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.\" Copyright (C) 2020 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\"
.TH SECCOMP_UNOTIFY 2 2020-10-01 "Linux" "Linux Programmer's Manual"
.SH NAME
seccomp_unotify \- Seccomp user-space notification mechanism
.SH SYNOPSIS
.nf
.B #include <linux/seccomp.h>
.B #include <linux/filter.h>
.B #include <linux/audit.h>
.PP
.BI "int seccomp(unsigned int " operation ", unsigned int " flags \
", void *" args );
.PP
.B #include <sys/ioctl.h>
.PP
.BI "int ioctl(int " fd ", SECCOMP_IOCTL_NOTIF_RECV,"
.BI " struct seccomp_notif *" req );
.BI "int ioctl(int " fd ", SECCOMP_IOCTL_NOTIF_SEND,"
.BI " struct seccomp_notif_resp *" resp );
.BI "int ioctl(int " fd ", SECCOMP_IOCTL_NOTIF_ID_VALID, __u64 *" id );
.fi
.SH DESCRIPTION
This page describes the user-space notification mechanism provided by the
Secure Computing (seccomp) facility.
As well as the use of the
.B SECCOMP_FILTER_FLAG_NEW_LISTENER
flag, the
.BR SECCOMP_RET_USER_NOTIF
action value, and the
.B SECCOMP_GET_NOTIF_SIZES
operation described in
.BR seccomp (2),
this mechanism involves the use of a number of related
.BR ioctl (2)
operations (described below).
.\"
.SS Overview
In conventional usage of a seccomp filter,
the decision about how to treat a system call is made by the filter itself.
By contrast, the user-space notification mechanism allows
the seccomp filter to delegate
the handling of the system call to another user-space process.
Note that this mechanism is explicitly
.B not
intended as a method implementing security policy; see NOTES.
.PP
In the discussion that follows,
the thread(s) on which the seccomp filter is installed is (are)
referred to as the
.IR target ,
and the process that is notified by the user-space notification
mechanism is referred to as the
.IR supervisor .
.PP
A suitably privileged supervisor can use the user-space notification
mechanism to perform actions on behalf of the target.
The advantage of the user-space notification mechanism is that
the supervisor will
usually be able to retrieve information about the target and the
performed system call that the seccomp filter itself cannot.
(A seccomp filter is limited in the information it can obtain and
the actions that it can perform because it
is running on a virtual machine inside the kernel.)
.PP
An overview of the steps performed by the target and the supervisor
is as follows:
.\"-------------------------------------
.IP 1. 3
The target establishes a seccomp filter in the usual manner,
but with two differences:
.RS
.IP \(bu 2
The
.BR seccomp (2)
.I flags
argument includes the flag
.BR SECCOMP_FILTER_FLAG_NEW_LISTENER .
Consequently, the return value of the (successful)
.BR seccomp (2)
call is a new "listening"
file descriptor that can be used to receive notifications.
Only one "listening" seccomp filter can be installed for a thread.
.\" FIXME
.\" Is the last sentence above correct?
.IP \(bu
In cases where it is appropriate, the seccomp filter returns the action value
.BR SECCOMP_RET_USER_NOTIF .
This return value will trigger a notification event.
.RE
.\"-------------------------------------
.IP 2.
In order that the supervisor can obtain notifications
using the listening file descriptor,
(a duplicate of) that file descriptor must be passed from
the target to the supervisor.
One way in which this could be done is by passing the file descriptor
over a UNIX domain socket connection between the target and the supervisor
(using the
.BR SCM_RIGHTS
ancillary message type described in
.BR unix (7)).
.\" Jann Horn:
.\" Instead of using unix domain sockets to send the fd to the
.\" parent, I think you could also use clone3() with
.\" flags==CLONE_FILES|SIGCHLD, dup2() the seccomp fd to an fd
.\" that was reserved in the parent, call unshare(CLONE_FILES)
.\" in the child after setting up the seccomp fd, and wake
.\" up the parent with something like pthread_cond_signal()?
.\" I'm not sure whether that'd look better or worse in the
.\" end though, so maybe just ignore this comment.
.\"-------------------------------------
.IP 3.
The supervisor will receive notification events
on the listening file descriptor.
These events are returned as structures of type
.IR seccomp_notif .
Because this structure and its size may evolve over kernel versions,
the supervisor must first determine the size of this structure
using the
.BR seccomp (2)
.B SECCOMP_GET_NOTIF_SIZES
operation, which returns a structure of type
.IR seccomp_notif_sizes .
The supervisor allocates a buffer of size
.I seccomp_notif_sizes.seccomp_notif
bytes to receive notification events.
In addition,the supervisor allocates another buffer of size
.I seccomp_notif_sizes.seccomp_notif_resp
bytes for the response (a
.I struct seccomp_notif_resp
structure)
that it will provide to the kernel (and thus the target).
.\"-------------------------------------
.IP 4.
The target then performs its workload,
which includes system calls that will be controlled by the seccomp filter.
Whenever one of these system calls causes the filter to return the
.B SECCOMP_RET_USER_NOTIF
action value, the kernel does
.I not
(yet) execute the system call;
instead, execution of the target is temporarily blocked inside
the kernel (in a sleep state that is interruptible by signals)
and a notification event is generated on the listening file descriptor.
.\"-------------------------------------
.IP 5.
The supervisor can now repeatedly monitor the
listening file descriptor for
.BR SECCOMP_RET_USER_NOTIF -triggered
events.
To do this, the supervisor uses the
.B SECCOMP_IOCTL_NOTIF_RECV
.BR ioctl (2)
operation to read information about a notification event;
this operation blocks until an event is available.
The operation returns a
.I seccomp_notif
structure containing information about the system call
that is being attempted by the target.
.\"-------------------------------------
.IP 6.
The
.I seccomp_notif
structure returned by the
.B SECCOMP_IOCTL_NOTIF_RECV
operation includes the same information (a
.I seccomp_data
structure) that was passed to the seccomp filter.
This information allows the supervisor to discover the system call number and
the arguments for the target's system call.
In addition, the notification event contains the ID of the thread
that triggered the notification.
.IP
The information in the notification can be used to discover the
values of pointer arguments for the target's system call.
(This is something that can't be done from within a seccomp filter.)
One way in which the supervisor can do this is to open the corresponding
.I /proc/[tid]/mem
file (see
.BR proc (5))
and read bytes from the location that corresponds to one of
the pointer arguments whose value is supplied in the notification event.
.\" Tycho Andersen mentioned that there are alternatives to /proc/PID/mem,
.\" such as ptrace() and /proc/PID/map_files
(The supervisor must be careful to avoid
a race condition that can occur when doing this;
see the description of the
.BR SECCOMP_IOCTL_NOTIF_ID_VALID
.BR ioctl (2)
operation below.)
In addition,
the supervisor can access other system information that is visible
in user space but which is not accessible from a seccomp filter.
.\"-------------------------------------
.IP 7.
Having obtained information as per the previous step,
the supervisor may then choose to perform an action in response
to the target's system call
(which, as noted above, is not executed when the seccomp filter returns the
.B SECCOMP_RET_USER_NOTIF
action value).
.IP
One example use case here relates to containers.
The target may be located inside a container where
it does not have sufficient capabilities to mount a filesystem
in the container's mount namespace.
However, the supervisor may be a more privileged process that
does have sufficient capabilities to perform the mount operation.
.\"-------------------------------------
.IP 8.
The supervisor then sends a response to the notification.
The information in this response is used by the kernel to construct
a return value for the target's system call and provide
a value that will be assigned to the
.I errno
variable of the target.
.IP
The response is sent using the
.B SECCOMP_IOCTL_NOTIF_SEND
.BR ioctl (2)
operation, which is used to transmit a
.I seccomp_notif_resp
structure to the kernel.
This structure includes a cookie value that the supervisor obtained in the
.I seccomp_notif
structure returned by the
.B SECCOMP_IOCTL_NOTIF_RECV
operation.
This cookie value allows the kernel to associate the response with the
target.
.\"-------------------------------------
.IP 9.
Once the notification has been sent,
the system call in the target thread unblocks,
returning the information that was provided by the supervisor
in the notification response.
.\"-------------------------------------
.PP
As a variation on the last two steps,
the supervisor can send a response that tells the kernel that it
should execute the target thread's system call; see the discussion of
.BR SECCOMP_USER_NOTIF_FLAG_CONTINUE ,
below.
.\"
.SS ioctl(2) operations
The following
.BR ioctl (2)
operations are provided to support seccomp user-space notification.
For each of these operations, the first (file descriptor) argument of
.BR ioctl (2)
is the listening file descriptor returned by a call to
.BR seccomp (2)
with the
.BR SECCOMP_FILTER_FLAG_NEW_LISTENER
flag.
.TP
.BR SECCOMP_IOCTL_NOTIF_RECV " (since Linux 5.0)"
This operation is used to obtain a user-space
notification event.
If no such event is currently pending,
the operation blocks until an event occurs.
The third
.BR ioctl (2)
argument is a pointer to a structure of the following form
which contains information about the event.
This structure must be zeroed out before the call.
.IP
.in +4n
.EX
struct seccomp_notif {
__u64 id; /* Cookie */
__u32 pid; /* TID of target thread */
__u32 flags; /* Currently unused (0) */
struct seccomp_data data; /* See seccomp(2) */
};
.EE
.in
.IP
The fields in this structure are as follows:
.RS
.TP
.I id
This is a cookie for the notification.
Each such cookie is guaranteed to be unique for the corresponding
seccomp filter.
.RS
.IP \(bu 2
It can be used with the
.B SECCOMP_IOCTL_NOTIF_ID_VALID
.BR ioctl (2)
operation to verify that the target is still alive.
.IP \(bu
When returning a notification response to the kernel,
the supervisor must include the cookie value in the
.IR seccomp_notif_resp
structure that is specified as the argument of the
.BR SECCOMP_IOCTL_NOTIF_SEND
operation.
.RE
.TP
.I pid
This is the thread ID of the target thread that triggered
the notification event.
.TP
.I flags
This is a bit mask of flags providing further information on the event.
In the current implementation, this field is always zero.
.TP
.I data
This is a
.I seccomp_data
structure containing information about the system call that
triggered the notification.
This is the same structure that is passed to the seccomp filter.
See
.BR seccomp (2)
for details of this structure.
.RE
.IP
On success, this operation returns 0; on failure, \-1 is returned, and
.I errno
is set to indicate the cause of the error.
This operation can fail with the following errors:
.RS
.TP
.BR EINVAL " (since Linux 5.5)"
.\" commit 2882d53c9c6f3b8311d225062522f03772cf0179
The
.I seccomp_notif
structure that was passed to the call contained nonzero fields.
.TP
.B ENOENT
The target thread was killed by a signal as the notification information
was being generated,
or the target's (blocked) system call was interrupted by a signal handler.
.RE
.\" FIXME
.\" From my experiments,
.\" it appears that if a SECCOMP_IOCTL_NOTIF_RECV is done after
.\" the target thread terminates, then the ioctl() simply
.\" blocks (rather than returning an error to indicate that the
.\" target no longer exists).
.\"
.\" I found that surprising, and it required some contortions in
.\" the example program. It was not possible to code my SIGCHLD
.\" handler (which reaps the zombie when the worker/target
.\" terminates) to simply set a flag checked in the main
.\" handleNotifications() loop, since this created an
.\" unavoidable race where the child might terminate just after
.\" I had checked the flag, but before I blocked (forever!) in the
.\" SECCOMP_IOCTL_NOTIF_RECV operation. Instead, I had to code
.\" the signal handler to simply call _exit(2) in order to
.\" terminate the parent process (the supervisor).
.\"
.\" Is this expected behavior? It seems to me rather
.\" desirable that SECCOMP_IOCTL_NOTIF_RECV should give an error
.\" if the target has terminated.
.\"
.\" For now, this behavior is documented in BUGS.
.TP
.BR SECCOMP_IOCTL_NOTIF_ID_VALID " (since Linux 5.0)"
This operation can be used to check that a notification ID
returned by an earlier
.B SECCOMP_IOCTL_NOTIF_RECV
operation is still valid (i.e., that the target still exists).
.IP
The third
.BR ioctl (2)
argument is a pointer to the cookie
.RI ( id )
returned by the
.B SECCOMP_IOCTL_NOTIF_RECV
operation.
.IP
This operation is necessary to avoid race conditions that can occur when the
.I pid
returned by the
.B SECCOMP_IOCTL_NOTIF_RECV
operation terminates, and that process ID is reused by another process.
An example of this kind of race is the following
.RS
.IP 1. 3
A notification is generated on the listening file descriptor.
The returned
.I seccomp_notif
contains the TID of the target thread (in the
.I pid
field of the structure).
.IP 2.
The target terminates.
.IP 3.
Another thread or process is created on the system that by chance reuses the
TID that was freed when the target terminated.
.IP 4.
The supervisor
.BR open (2)s
the
.IR /proc/[tid]/mem
file for the TID obtained in step 1, with the intention of (say)
inspecting the memory location(s) that containing the argument(s) of
the system call that triggered the notification in step 1.
.RE
.IP
In the above scenario, the risk is that the supervisor may try
to access the memory of a process other than the target.
This race can be avoided by following the call to
.BR open (2)
with a
.B SECCOMP_IOCTL_NOTIF_ID_VALID
operation to verify that the process that generated the notification
is still alive.
(Note that if the target terminates after the latter step,
a subsequent
.BR read (2)
from the file descriptor may return 0, indicating end of file.)
.\" Jann Horn:
.\" the PID can be reused, but the /proc/$pid directory is
.\" internally not associated with the numeric PID, but,
.\" conceptually speaking, with a specific incarnation of the
.\" PID, or something like that. (Actually, it is associated
.\" with the "struct pid", which is not reused, instead of the
.\" numeric PID.
.IP
On success (i.e., the notification ID is still valid),
this operation returns 0.
On failure (i.e., the notification ID is no longer valid),
\-1 is returned, and
.I errno
is set to
.BR ENOENT .
.TP
.BR SECCOMP_IOCTL_NOTIF_SEND " (since Linux 5.0)"
This operation is used to send a notification response back to the kernel.
The third
.BR ioctl (2)
argument of this structure is a pointer to a structure of the following form:
.IP
.in +4n
.EX
struct seccomp_notif_resp {
__u64 id; /* Cookie value */
__s64 val; /* Success return value */
__s32 error; /* 0 (success) or negative
error number */
__u32 flags; /* See below */
};
.EE
.in
.IP
The fields of this structure are as follows:
.RS
.TP
.I id
This is the cookie value that was obtained using the
.B SECCOMP_IOCTL_NOTIF_RECV
operation.
This cookie value allows the kernel to correctly associate this response
with the system call that triggered the user-space notification.
.TP
.I val
This is the value that will be used for a spoofed
success return for the target's system call; see below.
.TP
.I error
This is the value that will be used as the error number
.RI ( errno )
for a spoofed error return for the target's system call; see below.
.TP
.I flags
This is a bit mask that includes zero or more of the following flags:
.RS
.TP
.BR SECCOMP_USER_NOTIF_FLAG_CONTINUE " (since Linux 5.5)"
Tell the kernel to execute the target's system call.
.\" commit fb3c5386b382d4097476ce9647260fc89b34afdb
.RE
.RE
.IP
Two kinds of response are possible:
.RS
.IP \(bu 2
A response to the kernel telling it to execute the
target's system call.
In this case, the
.I flags
field includes
.B SECCOMP_USER_NOTIF_FLAG_CONTINUE
and the
.I error
and
.I val
fields must be zero.
.IP
This kind of response can be useful in cases where the supervisor needs
to do deeper analysis of the target's system call than is possible
from a seccomp filter (e.g., examining the values of pointer arguments),
and, having decided that the system call does not require emulation
by the supervisor, the supervisor wants the system call to
be executed normally in the target.
.IP
The
.B SECCOMP_USER_NOTIF_FLAG_CONTINUE
flag should be used with caution; see NOTES.
.IP \(bu
A spoofed return value for the target's system call.
In this case, the kernel does not execute the target's system call,
instead causing the system call to return a spoofed value as specified by
fields of the
.I seccomp_notif_resp
structure.
The supervisor should set the fields of this structure as follows:
.RS
.IP + 3
.I flags
does not contain
.BR SECCOMP_USER_NOTIF_FLAG_CONTINUE .
.IP +
.I error
is set either to 0 for a spoofed "success" return or to a negative
error number for a spoofed "failure" return.
In the former case, the kernel causes the target's system call
to return the value specified in the
.I val
field.
In the later case, the kernel causes the target's system call
to return \-1, and
.I errno
is assigned the negated
.I error
value.
.IP +
.I val
is set to a value that will be used as the return value for a spoofed
"success" return for the target's system call.
The value in this field is ignored if the
.I error
field contains a nonzero value.
.RE
.RE
.IP
On success, this operation returns 0; on failure, \-1 is returned, and
.I errno
is set to indicate the cause of the error.
This operation can fail with the following errors:
.RS
.TP
.B EINPROGRESS
A response to this notification has already been sent.
.TP
.B EINVAL
An invalid value was specified in the
.I flags field.
.TP
.B
.B EINVAL
The
.I flags
field contained
.BR SECCOMP_USER_NOTIF_FLAG_CONTINUE ,
and the
.I error
or
.I val
field was not zero.
.TP
.B ENOENT
The blocked system call in the target
has been interrupted by a signal handler
or the target has terminated.
.\" Jann Horn notes:
.\" you could also get this [ENOENT] if a response has already
.\" been sent, instead of EINPROGRESS - the only difference is
.\" whether the target thread has picked up the response yet
.RE
.SH NOTES
.SS select()/poll()/epoll semantics
The file descriptor returned when
.BR seccomp (2)
is employed with the
.B SECCOMP_FILTER_FLAG_NEW_LISTENER
flag can be monitored using
.BR poll (2),
.BR epoll (7),
and
.BR select (2).
These interfaces indicate that the file descriptor is ready as follows:
.IP \(bu 2
When a notification is pending,
these interfaces indicate that the file descriptor is readable.
Following such an indication, a subsequent
.B SECCOMP_IOCTL_NOTIF_RECV
.BR ioctl (2)
will not block, returning either information about a notification
or else failing with the error
.B EINTR
if the target has been killed by a signal or its system call
has been interrupted by a signal handler.
.IP \(bu
After the notification has been received (i.e., by the
.B SECCOMP_IOCTL_NOTIF_RECV
.BR ioctl (2)
operation), these interfaces indicate that the file descriptor is writable,
meaning that a notification response can be sent using the
.B SECCOMP_IOCTL_NOTIF_SEND
.BR ioctl (2)
operation.
.\" FIXME
.\" But (how) is the writable/(E)POLLOUT useful?
.IP \(bu
After the last thread using the filter has terminated and been reaped using
.BR waitpid (2)
(or similar),
the file descriptor indicates an end-of-file condition (readable in
.BR select (2);
.BR POLLHUP / EPOLLHUP
in
.BR poll (2)/
.BR epoll_wait (2)).
.SS Design goals; use of SECCOMP_USER_NOTIF_FLAG_CONTINUE
The intent of the user-space notification feature is
to allow system calls to be performed on behalf of the target.
The target's system call should either be handled by the supervisor or
allowed to continue normally in the kernel (where standard security
policies will be applied).
.PP
.BR "Note well" :
this mechanism must not be used to make security policy decisions
about the system call,
which would be inherently race-prone for reasons described next.
.PP
The
.B SECCOMP_USER_NOTIF_FLAG_CONTINUE
flag must be used with caution.
If set by the supervisor, the target's system call will continue.
However, there is a time-of-check, time-of-use race here,
since an attacker could exploit the interval of time where the target is
blocked waiting on the "continue" response to do things such as
rewriting the system call arguments.
.PP
Note furthermore that a user-space notifier can be bypassed if
the existing filters allow the use of
.BR seccomp (2)
or
.BR prctl (2)
to install a filter that returns an action value with a higher precedence than
.B SECCOMP_RET_USER_NOTIF
(see
.BR seccomp (2)).
.PP
It should thus be absolutely clear that the
seccomp user-space notification mechanism
.B can not
be used to implement a security policy!
It should only ever be used in scenarios where a more privileged process
supervises the system calls of a lesser privileged target to
get around kernel-enforced security restrictions when
the supervisor deems this safe.
In other words,
in order to continue a system call, the supervisor should be sure that
another security mechanism or the kernel itself will sufficiently block
the system call if its arguments are rewritten to something unsafe.
.\"
.SS Interaction with SA_RESTART signal handlers
Consider the following scenario:
.IP \(bu 2
The target process has used
.BR sigaction (2)
to install a signal handler with the
.B SA_RESTART
flag.
.IP \(bu
The target has made a system call that triggered a seccomp
user-space notification and the target is currently blocked
until the supervisor sends a notification response.
.IP \(bu
A signal is delivered to the target and the signal handler is executed.
.IP \(bu
When (if) the supervisor attempts to send a notification response, the
.B SECCOMP_IOCTL_NOTIF_SEND
.BR ioctl (2))
operation will fail with the
.BR ENOENT
error.
.PP
In this scenario, the kernel will restart the target's system call.
Consequently, the supervisor will receive another user-space notification.
Thus, depending on how many times the blocked system call
is interrupted by a signal handler,
the supervisor may receive multiple notifications for
the same system call in the target.
.PP
One oddity is that system call restarting as described in this scenario
will occur even for the blocking system calls listed in
.BR signal (7)
that would
.B never
normally be restarted by the
.BR SA_RESTART
flag.
.SH BUGS
If a
.BR SECCOMP_IOCTL_NOTIF_RECV
.BR ioctl (2)
operation
.\" or a poll/epoll/select
is performed after the target terminates, then the
.BR ioctl (2)
call simply blocks (rather than returning an error to indicate that the
target no longer exists).
.SH EXAMPLES
The (somewhat contrived) program shown below demonstrates the use of
the interfaces described in this page.
The program creates a child process that serves as the "target" process.
The child process installs a seccomp filter that returns the
.B SECCOMP_RET_USER_NOTIF
action value if a call is made to
.BR mkdir (2).
The child process then calls
.BR mkdir (2)
once for each of the supplied command-line arguments,
and reports the result returned by the call.
After processing all arguments, the child process terminates.
.PP
The parent process acts as the supervisor, listening for the notifications
that are generated when the target process calls
.BR mkdir (2).
When such a notification occurs,
the supervisor examines the memory of the target process (using
.IR /proc/[pid]/mem )
to discover the pathname argument that was supplied to the
.BR mkdir (2)
call, and performs one of the following actions:
.IP \(bu 2
If the pathname begins with the prefix "/tmp/",
then the supervisor attempts to create the specified directory,
and then spoofs a return for the target process based on the return
value of the supervisor's
.BR mkdir (2)
call.
In the event that that call succeeds,
the spoofed success return value is the length of the pathname.
.IP \(bu
If the pathname begins with "./" (i.e., it is a relative pathname),
the supervisor sends a
.B SECCOMP_USER_NOTIF_FLAG_CONTINUE
response to the kernel to say that the kernel should execute
the target process's
.BR mkdir (2)
call.
.IP \(bu
If the pathname begins with some other prefix,
the supervisor spoofs an error return for the target process,
so that the target process's
.BR mkdir (2)
call appears to fail with the error
.BR EOPNOTSUPP
("Operation not supported").
Additionally, if the specified pathname is exactly "/bye",
then the supervisor terminates.
.PP
This program can be used to demonstrate various aspects of the
behavior of the seccomp user-space notification mechanism.
To help aid such demonstrations,
the program logs various messages to show the operation
of the target process (lines prefixed "T:") and the supervisor
(indented lines prefixed "S:").
.PP
In the following example, the target attempts to create the directory
.IR /tmp/x .
Upon receiving the notification, the supervisor creates the directory on the
target's behalf,
and spoofs a success return to be received by the target process's
.BR mkdir (2)
call.
.PP
.in +4n
.EX
$ \fB./seccomp_unotify /tmp/x\fP
T: PID = 23168
T: about to mkdir("/tmp/x")
S: got notification (ID 0x17445c4a0f4e0e3c) for PID 23168
S: executing: mkdir("/tmp/x", 0700)
S: success! spoofed return = 6
S: sending response (flags = 0; val = 6; error = 0)
T: SUCCESS: mkdir(2) returned 6
T: terminating
S: target has terminated; bye
.EE
.in
.PP
In the above output, note that the spoofed return value seen by the target
process is 6 (the length of the pathname
.IR /tmp/x ),
whereas a normal
.BR mkdir (2)
call returns 0 on success.
.PP
In the next example, the target attempts to create a directory using the
relative pathname
.IR ./sub .
Since this pathname starts with "./",
the supervisor sends a
.B SECCOMP_USER_NOTIF_FLAG_CONTINUE
response to the kernel,
and the kernel then (successfully) executes the target process's
.BR mkdir (2)
call.
.PP
.in +4n
.EX
$ \fB./seccomp_unotify ./sub\fP
T: PID = 23204
T: about to mkdir("./sub")
S: got notification (ID 0xddb16abe25b4c12) for PID 23204
S: target can execute system call
S: sending response (flags = 0x1; val = 0; error = 0)
T: SUCCESS: mkdir(2) returned 0
T: terminating
S: target has terminated; bye
.EE
.in
.PP
If the target process attempts to create a directory with
a pathname that doesn't start with "." and doesn't begin with the prefix
"/tmp/", then the supervisor spoofs an error return
.RB ( EOPNOTSUPP ,
"Operation not supported")
for the target's
.BR mkdir (2)
call (which is not executed):
.PP
.in +4n
.EX
$ \fB./seccomp_unotify /xxx\fP
T: PID = 23178
T: about to mkdir("/xxx")
S: got notification (ID 0xe7dc095d1c524e80) for PID 23178
S: spoofing error response (Operation not supported)
S: sending response (flags = 0; val = 0; error = \-95)
T: ERROR: mkdir(2): Operation not supported
T: terminating
S: target has terminated; bye
.EE
.in
.PP
In the next example,
the target process attempts to create a directory with the pathname
.BR /tmp/nosuchdir/b .
Upon receiving the notification,
the supervisor attempts to create that directory, but the
.BR mkdir (2)
call fails because the directory
.BR /tmp/nosuchdir
does not exist.
Consequently, the supervisor spoofs an error return that passes the error
that it received back to the target process's
.BR mkdir (2)
call.
.PP
.in +4n
.EX
$ \fB./seccomp_unotify /tmp/nosuchdir/b\fP
T: PID = 23199
T: about to mkdir("/tmp/nosuchdir/b")
S: got notification (ID 0x8744454293506046) for PID 23199
S: executing: mkdir("/tmp/nosuchdir/b", 0700)
S: failure! (errno = 2; No such file or directory)
S: sending response (flags = 0; val = 0; error = \-2)
T: ERROR: mkdir(2): No such file or directory
T: terminating
S: target has terminated; bye
.EE
.in
.PP
If the supervisor receives a notification and sees that the
argument of the target's
.BR mkdir (2)
is the string "/bye", then (as well as spoofing an
.B EOPNOTSUPP
error), the supervisor terminates.
If the target process subsequently executes another
.BR mkdir (2)
that triggers its seccomp filter to return the
.B SECCOMP_RET_USER_NOTIF
action value, then the kernel causes the target process's system call to
fail with the error
.B ENOSYS
("Function not implemented").
This is demonstrated by the following example:
.PP
.in +4n
.EX
$ \fB./seccomp_unotify /bye /tmp/y\fP
T: PID = 23185
T: about to mkdir("/bye")
S: got notification (ID 0xa81236b1d2f7b0f4) for PID 23185
S: spoofing error response (Operation not supported)
S: sending response (flags = 0; val = 0; error = \-95)
S: terminating **********
T: ERROR: mkdir(2): Operation not supported
T: about to mkdir("/tmp/y")
T: ERROR: mkdir(2): Function not implemented
T: terminating
.EE
.in
.\"
.SS Program source
.EX
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <linux/audit.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <signal.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/un.h>
#include <sys/syscall.h>
#include <unistd.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \e
} while (0)
/* Send the file descriptor \(aqfd\(aq over the connected UNIX domain socket
\(aqsockfd\(aq. Returns 0 on success, or \-1 on error. */
static int
sendfd(int sockfd, int fd)
{
struct msghdr msgh;
struct iovec iov;
int data;
struct cmsghdr *cmsgp;
/* Allocate a char array of suitable size to hold the ancillary data.
However, since this buffer is in reality a \(aqstruct cmsghdr\(aq, use a
union to ensure that it is suitably aligned. */
union {
char buf[CMSG_SPACE(sizeof(int))];
/* Space large enough to hold an \(aqint\(aq */
struct cmsghdr align;
} controlMsg;
/* The \(aqmsg_name\(aq field can be used to specify the address of the
destination socket when sending a datagram. However, we do not
need to use this field because \(aqsockfd\(aq is a connected socket. */
msgh.msg_name = NULL;
msgh.msg_namelen = 0;
/* On Linux, we must transmit at least one byte of real data in
order to send ancillary data. We transmit an arbitrary integer
whose value is ignored by recvfd(). */
msgh.msg_iov = &iov;
msgh.msg_iovlen = 1;
iov.iov_base = &data;
iov.iov_len = sizeof(int);
data = 12345;
/* Set \(aqmsghdr\(aq fields that describe ancillary data */
msgh.msg_control = controlMsg.buf;
msgh.msg_controllen = sizeof(controlMsg.buf);
/* Set up ancillary data describing file descriptor to send */
cmsgp = CMSG_FIRSTHDR(&msgh);
cmsgp\->cmsg_level = SOL_SOCKET;
cmsgp\->cmsg_type = SCM_RIGHTS;
cmsgp\->cmsg_len = CMSG_LEN(sizeof(int));
memcpy(CMSG_DATA(cmsgp), &fd, sizeof(int));
/* Send real plus ancillary data */
if (sendmsg(sockfd, &msgh, 0) == \-1)
return \-1;
return 0;
}
/* Receive a file descriptor on a connected UNIX domain socket. Returns
the received file descriptor on success, or \-1 on error. */
static int
recvfd(int sockfd)
{
struct msghdr msgh;
struct iovec iov;
int data, fd;
ssize_t nr;
/* Allocate a char buffer for the ancillary data. See the comments
in sendfd() */
union {
char buf[CMSG_SPACE(sizeof(int))];
struct cmsghdr align;
} controlMsg;
struct cmsghdr *cmsgp;
/* The \(aqmsg_name\(aq field can be used to obtain the address of the
sending socket. However, we do not need this information. */
msgh.msg_name = NULL;
msgh.msg_namelen = 0;
/* Specify buffer for receiving real data */
msgh.msg_iov = &iov;
msgh.msg_iovlen = 1;
iov.iov_base = &data; /* Real data is an \(aqint\(aq */
iov.iov_len = sizeof(int);
/* Set \(aqmsghdr\(aq fields that describe ancillary data */
msgh.msg_control = controlMsg.buf;
msgh.msg_controllen = sizeof(controlMsg.buf);
/* Receive real plus ancillary data; real data is ignored */
nr = recvmsg(sockfd, &msgh, 0);
if (nr == \-1)
return \-1;
cmsgp = CMSG_FIRSTHDR(&msgh);
/* Check the validity of the \(aqcmsghdr\(aq */
if (cmsgp == NULL ||
cmsgp\->cmsg_len != CMSG_LEN(sizeof(int)) ||
cmsgp\->cmsg_level != SOL_SOCKET ||
cmsgp\->cmsg_type != SCM_RIGHTS) {
errno = EINVAL;
return \-1;
}
/* Return the received file descriptor to our caller */
memcpy(&fd, CMSG_DATA(cmsgp), sizeof(int));
return fd;
}
static void
sigchldHandler(int sig)
{
char *msg = "\etS: target has terminated; bye\en";
write(STDOUT_FILENO, msg, strlen(msg));
_exit(EXIT_SUCCESS);
}
static int
seccomp(unsigned int operation, unsigned int flags, void *args)
{
return syscall(__NR_seccomp, operation, flags, args);
}
/* The following is the x86\-64\-specific BPF boilerplate code for checking
that the BPF program is running on the right architecture + ABI. At
completion of these instructions, the accumulator contains the system
call number. */
/* For the x32 ABI, all system call numbers have bit 30 set */
#define X32_SYSCALL_BIT 0x40000000
#define X86_64_CHECK_ARCH_AND_LOAD_SYSCALL_NR \e
BPF_STMT(BPF_LD | BPF_W | BPF_ABS, \e
(offsetof(struct seccomp_data, arch))), \e
BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, AUDIT_ARCH_X86_64, 0, 2), \e
BPF_STMT(BPF_LD | BPF_W | BPF_ABS, \e
(offsetof(struct seccomp_data, nr))), \e
BPF_JUMP(BPF_JMP | BPF_JGE | BPF_K, X32_SYSCALL_BIT, 0, 1), \e
BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS)
/* installNotifyFilter() installs a seccomp filter that generates
user\-space notifications (SECCOMP_RET_USER_NOTIF) when the process
calls mkdir(2); the filter allows all other system calls.
The function return value is a file descriptor from which the
user\-space notifications can be fetched. */
static int
installNotifyFilter(void)
{
struct sock_filter filter[] = {
X86_64_CHECK_ARCH_AND_LOAD_SYSCALL_NR,
/* mkdir() triggers notification to user\-space supervisor */
BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_mkdir, 0, 1),
BPF_STMT(BPF_RET + BPF_K, SECCOMP_RET_USER_NOTIF),
/* Every other system call is allowed */
BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
};
struct sock_fprog prog = {
.len = sizeof(filter) / sizeof(filter[0]),
.filter = filter,
};
/* Install the filter with the SECCOMP_FILTER_FLAG_NEW_LISTENER flag;
as a result, seccomp() returns a notification file descriptor. */
int notifyFd = seccomp(SECCOMP_SET_MODE_FILTER,
SECCOMP_FILTER_FLAG_NEW_LISTENER, &prog);
if (notifyFd == \-1)
errExit("seccomp\-install\-notify\-filter");
return notifyFd;
}
/* Close a pair of sockets created by socketpair() */
static void
closeSocketPair(int sockPair[2])
{
if (close(sockPair[0]) == \-1)
errExit("closeSocketPair\-close\-0");
if (close(sockPair[1]) == \-1)
errExit("closeSocketPair\-close\-1");
}
/* Implementation of the target process; create a child process that:
(1) installs a seccomp filter with the
SECCOMP_FILTER_FLAG_NEW_LISTENER flag;
(2) writes the seccomp notification file descriptor returned from
the previous step onto the UNIX domain socket, \(aqsockPair[0]\(aq;
(3) calls mkdir(2) for each element of \(aqargv\(aq.
The function return value in the parent is the PID of the child
process; the child does not return from this function. */
static pid_t
targetProcess(int sockPair[2], char *argv[])
{
pid_t targetPid = fork();
if (targetPid == \-1)
errExit("fork");
if (targetPid > 0) /* In parent, return PID of child */
return targetPid;
/* Child falls through to here */
printf("T: PID = %ld\en", (long) getpid());
/* Install seccomp filter(s) */
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
errExit("prctl");
int notifyFd = installNotifyFilter();
/* Pass the notification file descriptor to the tracing process over
a UNIX domain socket */
if (sendfd(sockPair[0], notifyFd) == \-1)
errExit("sendfd");
/* Notification and socket FDs are no longer needed in target */
if (close(notifyFd) == \-1)
errExit("close\-target\-notify\-fd");
closeSocketPair(sockPair);
/* Perform a mkdir() call for each of the command\-line arguments */
for (char **ap = argv; *ap != NULL; ap++) {
printf("\enT: about to mkdir(\e"%s\e")\en", *ap);
int s = mkdir(*ap, 0700);
if (s == \-1)
perror("T: ERROR: mkdir(2)");
else
printf("T: SUCCESS: mkdir(2) returned %d\en", s);
}
printf("\enT: terminating\en");
exit(EXIT_SUCCESS);
}
/* Check that the notification ID provided by a SECCOMP_IOCTL_NOTIF_RECV
operation is still valid. It will no longer be valid if the process
has terminated. This operation can be used when accessing /proc/PID
files in the target process in order to avoid TOCTOU race conditions
where the PID that is returned by SECCOMP_IOCTL_NOTIF_RECV terminates
and is reused by another process. */
static void
checkNotificationIdIsValid(int notifyFd, uint64_t id)
{
if (ioctl(notifyFd, SECCOMP_IOCTL_NOTIF_ID_VALID, &id) == \-1) {
fprintf(stderr, "\etS: notification ID check: "
"target has terminated!!!\en");
exit(EXIT_FAILURE);
}
}
/* Access the memory of the target process in order to discover the
pathname that was given to mkdir() */
static bool
getTargetPathname(struct seccomp_notif *req, int notifyFd,
char *path, size_t len)
{
char procMemPath[PATH_MAX];
snprintf(procMemPath, sizeof(procMemPath), "/proc/%d/mem", req\->pid);
int procMemFd = open(procMemPath, O_RDONLY);
if (procMemFd == \-1)
errExit("Supervisor: open");
/* Check that the process whose info we are accessing is still alive.
If the SECCOMP_IOCTL_NOTIF_ID_VALID operation (performed
in checkNotificationIdIsValid()) succeeds, we know that the
/proc/PID/mem file descriptor that we opened corresponds to the
process for which we received a notification. If that process
subsequently terminates, then read() on that file descriptor
will return 0 (EOF). */
checkNotificationIdIsValid(notifyFd, req\->id);
/* Read bytes at the location containing the pathname argument
(i.e., the first argument) of the mkdir(2) call */
ssize_t nread = pread(procMemFd, path, len, req\->data.args[0]);
if (nread == \-1)
errExit("pread");
if (nread == 0) {
fprintf(stderr, "\etS: pread() of /proc/PID/mem "
"returned 0 (EOF)\en");
exit(EXIT_FAILURE);
}
if (close(procMemFd) == \-1)
errExit("close\-/proc/PID/mem");
/* We have no guarantees about what was in the memory of the target
process. We therefore treat the buffer returned by pread() as
untrusted input. The buffer should be terminated by a null byte;
if not, then we will trigger an error for the target process. */
for (int j = 0; j < nread; j++)
if (path[j] == \(aq\0\(aq)
return true;
return false;
}
/* Handle notifications that arrive via the SECCOMP_RET_USER_NOTIF file
descriptor, \(aqnotifyFd\(aq. */
static void
handleNotifications(int notifyFd)
{
struct seccomp_notif_sizes sizes;
char path[PATH_MAX];
/* Discover the sizes of the structures that are used to receive
notifications and send notification responses, and allocate
buffers of those sizes. */
if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) == \-1)
errExit("\etS: seccomp\-SECCOMP_GET_NOTIF_SIZES");
struct seccomp_notif *req = malloc(sizes.seccomp_notif);
if (req == NULL)
errExit("\etS: malloc");
/* When allocating the response buffer, we must allow for the fact
that the user\-space binary may have been built with user\-space
headers where \(aqstruct seccomp_notif_resp\(aq is bigger than the
response buffer expected by the (older) kernel. Therefore, we
allocate a buffer that is the maximum of the two sizes. This
ensures that if the supervisor places bytes into the response
structure that are past the response size that the kernel expects,
then the supervisor is not touching an invalid memory location. */
size_t resp_size = sizes.seccomp_notif_resp;
if (sizeof(struct seccomp_notif_resp) > resp_size)
resp_size = sizeof(struct seccomp_notif_resp);
struct seccomp_notif_resp *resp = malloc(resp_size);
if (resp == NULL)
errExit("\etS: malloc");
/* Loop handling notifications */
for (;;) {
/* Wait for next notification, returning info in \(aq*req\(aq */
memset(req, 0, sizes.seccomp_notif);
if (ioctl(notifyFd, SECCOMP_IOCTL_NOTIF_RECV, req) == \-1) {
if (errno == EINTR)
continue;
errExit("Supervisor: ioctl\-SECCOMP_IOCTL_NOTIF_RECV");
}
printf("\etS: got notification (ID %#llx) for PID %d\en",
req\->id, req\->pid);
/* The only system call that can generate a notification event
is mkdir(2). Nevertheless, we check that the notified system
call is indeed mkdir() as kind of future\-proofing of this
code in case the seccomp filter is later modified to
generate notifications for other system calls. */
if (req\->data.nr != __NR_mkdir) {
printf("\etS: notification contained unexpected "
"system call number; bye!!!\en");
exit(EXIT_FAILURE);
}
bool pathOK = getTargetPathname(req, notifyFd, path,
sizeof(path));
/* Prepopulate some fields of the response */
resp\->id = req\->id; /* Response includes notification ID */
resp\->flags = 0;
resp\->val = 0;
/* If the target pathname was not valid, trigger an EINVAL error;
if the directory is in /tmp, then create it on behalf of the
supervisor; if the pathname starts with '.', tell the kernel
to let the target process execute the mkdir(); otherwise, give
an error for a directory pathname in any other location. */
if (!pathOK) {
resp->error = -EINVAL;
printf("\etS: spoofing error for invalid pathname (%s)\en",
strerror(-resp->error));
} else if (strncmp(path, "/tmp/", strlen("/tmp/")) == 0) {
printf("\etS: executing: mkdir(\e"%s\e", %#llo)\en",
path, req\->data.args[1]);
if (mkdir(path, req\->data.args[1]) == 0) {
resp\->error = 0; /* "Success" */
resp\->val = strlen(path); /* Used as return value of
mkdir() in target */
printf("\etS: success! spoofed return = %lld\en",
resp\->val);
} else {
/* If mkdir() failed in the supervisor, pass the error
back to the target */
resp\->error = \-errno;
printf("\etS: failure! (errno = %d; %s)\en", errno,
strerror(errno));
}
} else if (strncmp(path, "./", strlen("./")) == 0) {
resp\->error = resp\->val = 0;
resp\->flags = SECCOMP_USER_NOTIF_FLAG_CONTINUE;
printf("\etS: target can execute system call\en");
} else {
resp\->error = \-EOPNOTSUPP;
printf("\etS: spoofing error response (%s)\en",
strerror(\-resp\->error));
}
/* Send a response to the notification */
printf("\etS: sending response "
"(flags = %#x; val = %lld; error = %d)\en",
resp\->flags, resp\->val, resp\->error);
if (ioctl(notifyFd, SECCOMP_IOCTL_NOTIF_SEND, resp) == \-1) {
if (errno == ENOENT)
printf("\etS: response failed with ENOENT; "
"perhaps target process\(aqs syscall was "
"interrupted by a signal?\en");
else
perror("ioctl\-SECCOMP_IOCTL_NOTIF_SEND");
}
/* If the pathname is just "/bye", then the supervisor
terminates. This allows us to see what happens if the
target process makes further calls to mkdir(2). */
if (strcmp(path, "/bye") == 0) {
printf("\etS: terminating **********\en");
exit(EXIT_FAILURE);
}
}
}
/* Implementation of the supervisor process:
(1) obtains the notification file descriptor from \(aqsockPair[1]\(aq
(2) handles notifications that arrive on that file descriptor. */
static void
supervisor(int sockPair[2])
{
int notifyFd = recvfd(sockPair[1]);
if (notifyFd == \-1)
errExit("recvfd");
closeSocketPair(sockPair); /* We no longer need the socket pair */
handleNotifications(notifyFd);
}
int
main(int argc, char *argv[])
{
int sockPair[2];
setbuf(stdout, NULL);
if (argc < 2) {
fprintf(stderr, "At least one pathname argument is required\en");
exit(EXIT_FAILURE);
}
/* Create a UNIX domain socket that is used to pass the seccomp
notification file descriptor from the target process to the
supervisor process. */
if (socketpair(AF_UNIX, SOCK_STREAM, 0, sockPair) == \-1)
errExit("socketpair");
/* Create a child process\-\-the "target"\-\-that installs seccomp
filtering. The target process writes the seccomp notification
file descriptor onto \(aqsockPair[0]\(aq and then calls mkdir(2) for
each directory in the command\-line arguments. */
(void) targetProcess(sockPair, &argv[optind]);
/* Catch SIGCHLD when the target terminates, so that the
supervisor can also terminate. */
struct sigaction sa;
sa.sa_handler = sigchldHandler;
sa.sa_flags = 0;
sigemptyset(&sa.sa_mask);
if (sigaction(SIGCHLD, &sa, NULL) == \-1)
errExit("sigaction");
supervisor(sockPair);
exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR ioctl (2),
.BR seccomp (2)
.PP
A further example program can be found in the kernel source file
.IR samples/seccomp/user-trap.c .
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