.\" Copyright (C) 2014 Kees Cook .\" and Copyright (C) 2012 Will Drewry .\" and Copyright (C) 2008, 2014 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .TH SECCOMP 2 2014-06-23 "Linux" "Linux Programmer's Manual" .SH NAME seccomp \- operate on Secure Computing state of the process .SH SYNOPSIS .nf .B #include .B #include .B #include .B #include .\" FIXME Is sys/ptrace.h really required? It is not used in .\" the example program below. .B #include .BI "int seccomp(unsigned int " operation ", unsigned int " flags \ ", void *" args ); .fi .SH DESCRIPTION The .BR seccomp () system call operates on the Secure Computing (seccomp) state of the calling process. .\" FIXME: This page various uses the terms "process', "thread" and "task". .\" Probably only one of these (not "task"!) should be used in all .\" cases. I suspect it should be "thread". Currently, Linux supports the following .IR operation values: .TP .BR SECCOMP_SET_MODE_STRICT The only system calls that the thread is permitted to make are .BR read (2), .BR write (2), .BR _exit (2), and .BR sigreturn (2). Other system calls result in the delivery of a .BR SIGKILL signal Strict secure computing mode is useful for number-crunching applications that may need to execute untrusted byte code, perhaps obtained by reading from a pipe or socket. This operation is available only if the kernel is configured with .BR CONFIG_SECCOMP enabled. The value of .IR flags must be 0, and .IR args must be NULL. This operation is functionally identical to the call: prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT); .TP .BR SECCOMP_SET_MODE_FILTER The system calls allowed are defined by a pointer to a Berkeley Packet Filter (BPF) passed via .IR args . This arguMent is a pointer to a .IR "struct\ sock_fprog" ; it can be designed to filter arbitrary system calls and system call arguments. If the filter is invalid, .BR seccomp () fails, returning .BR EACCESS in .IR errno . .\" FIXME I (mtk) reworded the following paragraph substantially. .\" Please check it. If .BR fork (2) or .BR clone (2) is allowed by the filter, any child processes will be constrained to the same filters and system calls as the parent. If .BR execve (2) is allowed by the filter, the filters and constraints on permitted system calls are preserved across an .BR execve (2). .\" FIXME I (mtk) reworded the following paragraph substantially. .\" Please check it. In order to use the .BR SECCOMP_SET_MODE_FILTER operation, either the caller must have the .BR CAP_SYS_ADMIN capability or the call must be preceded by the call: prctl(PR_SET_NO_NEW_PRIVS, 1); Otherwise, the .BR SECCOMP_SET_MODE_FILTER operation will fail and return .BR EACCES in .IR errno . This requirement ensures that an unprivileged process cannot apply a malicious filter and then invoke a set-user-ID or other privileged program using .BR execve (2), thus potentially compromising that program If .BR prctl (2) or .BR seccomp (2) is allowed by the attached filter, further filters may be added. This will increase evaluation time, but allows for further reduction of the attack surface during execution of a process. The .BR SECCOMP_SET_MODE_FILTER operation is available only if the kernel is configured with .BR CONFIG_SECCOMP_FILTER enabled. When .IR flags is 0, this operation is functionally identical to the call: prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, args); The recognized .IR flags are: .RS .TP .BR SECCOMP_FILTER_FLAG_TSYNC When adding a new filter, synchronize all other threads of the calling process to the same seccomp filter tree. A "filter tree" is the ordered list of filters attached to a thread. (Attaching identical filters in separate .BR seccomp () calls results in different filters from this perspective.) If any thread cannot synchronize to the same filter tree, the call will not attach the new seccomp filter, and will fail, returning the first thread ID found that cannot synchronize. Synchronization will fail if another thread is in .BR SECCOMP_MODE_STRICT or if it has attached new seccomp filters to itself, diverging from the calling thread's filter tree. .RE .SH FILTERS When adding filters via .BR SECCOMP_SET_MODE_FILTER , .IR args points to a filter program: .in +4n .nf struct sock_fprog { unsigned short len; /* Number of BPF instructions */ struct sock_filter *filter; }; .fi .in Each program must contain one or more BPF instructions: .in +4n .nf struct sock_filter { /* Filter block */ __u16 code; /* Actual filter code */ __u8 jt; /* Jump true */ __u8 jf; /* Jump false */ __u32 k; /* Generic multiuse field */ }; .fi .in When executing the instructions, the BPF program executes over the system call information made available via: .in +4n .nf struct seccomp_data { int nr; /* system call number */ __u32 arch; /* AUDIT_ARCH_* value */ __u64 instruction_pointer; /* CPU instruction pointer */ __u64 args[6]; /* up to 6 system call arguments */ }; .fi .in .\" FIXME I find the next piece a little hard to understand, so, .\" some questions: .\" * If there are multiple filters, in what order are they executed? .\" (The man page should probably detail the answer to this question.) .\" * If there are multiple filters, are they all always executed? .\" I assume not, but the notion that .\" "the return value for the evaluation of a given system call .\" will always use the value with the highest precedence" .\" implies that even that if one filter generates (say) .\" SECCOMP_RET_ERRNO, then further filters may still be executed, .\" including one that generates (say) the "higher priority" .\" SECCOMP_RET_KILL condition. .\" Can you clarify the above? .\" Andy Lutomirski: .\" All of them are executed. The precedence rules determine what happens .\" if the filters return different values. A seccomp filter returns one of the values listed below. If multiple filters exist, the return value for the evaluation of a given system call will always use the value with the highest precedence. (For example, .BR SECCOMP_RET_KILL will always take precedence.) In decreasing order order of precedence, the values that may be returned by a seccomp filter are: .TP .BR SECCOMP_RET_KILL Results in the task exiting immediately without executing the system call. The task terminates as though killed by a .B SIGSYS signal .RI ( not .BR SIGKILL ). .TP .BR SECCOMP_RET_TRAP Results in the kernel sending a .BR SIGSYS signal to the triggering task without executing the system call. .IR siginfo\->si_call_addr will show the address of the system call instruction, and .IR siginfo\->si_syscall and .IR siginfo\->si_arch will indicate which system call was attempted. The program counter will be as though the system call happened (i.e., it will not point to the system call instruction). The return value register will contain an architecture\-dependent value; if resuming execution, set it to something sensible. (The architecture dependency is because replacing it with .BR ENOSYS could overwrite some useful information.) .\" FIXME The following sentence is the first time that SECCOMP_RET_DATA .\" is mentioned. SECCOMP_RET_DATA needs to be described in this .\" man page. The .BR SECCOMP_RET_DATA portion of the return value will be passed as .IR si_errno . .BR SIGSYS triggered by seccomp will have the value .BR SYS_SECCOMP in the .IR si_code field. .TP .BR SECCOMP_RET_ERRNO .\" FIXME What does "the return value" refer to in the next sentence? .\" It is not obvious to me. .\" Andy Lutomirski: .\" The return value is the value returned by the BPF program. Results in the lower 16-bits of the return value being passed to user space as the .IR errno without executing the system call. .TP .BR SECCOMP_RET_TRACE When returned, this value will cause the kernel to attempt to notify a .BR ptrace (2)-based tracer prior to executing the system call. .\" FIXME I (mtk) reworded the following sentence substantially. .\" Please check it. If there is no tracer present, the system call is not executed and returns a failure status with .I errno set to .BR ENOSYS . A tracer will be notified if it requests .BR PTRACE_O_TRACESECCOMP using .IR ptrace(PTRACE_SETOPTIONS) . The tracer will be notified of a .BR PTRACE_EVENT_SECCOMP and the .BR SECCOMP_RET_DATA portion of the BPF program return value will be available to the tracer via .BR PTRACE_GETEVENTMSG . The tracer can skip the system call by changing the system call number to \-1. Alternatively, the tracer can change the system call requested by changing the system call to a valid system call number. If the tracer asks to skip the system call, then the system call will appear to return the value that the tracer puts in the return value register. The seccomp check will not be run again after the tracer is notified. (This means that seccomp-based sandboxes .B "must not" allow use of .BR ptrace (2)\(emeven of other sandboxed processes\(emwithout extreme care; ptracers can use this mechanism to escape.) .TP .BR SECCOMP_RET_ALLOW Results in the system call being executed. .PP If multiple filters exist, the return value for the evaluation of a given system call will always use the highest precedent value. .\" FIXME The following sentence is the first time that SECCOMP_RET_ACTION .\" is mentioned. SECCOMP_RET_ACTION needs to be described in this .\" man page. Precedence is determined using only the .BR SECCOMP_RET_ACTION mask. When multiple filters return values of the same precedence, only the .BR SECCOMP_RET_DATA from the most recently installed filter will be returned. .SH RETURN VALUE On success, .BR seccomp () returns 0. On error, if .BR SECCOMP_FILTER_FLAG_TSYNC was used, the return value is the thread ID that caused the synchronization failure. On other errors, \-1 is returned, and .IR errno is set to indicate the cause of the error. .SH ERRORS .BR seccomp () can fail for the following reasons: .TP .BR EACCESS The caller did not have the .BR CAP_SYS_ADMIN capability, or had not set .IR no_new_privs before using .BR SECCOMP_SET_MODE_FILTER . .TP .BR EFAULT .IR args was required to be a valid address. .TP .BR EINVAL .IR operation is unknown; or .IR flags are invalid for the given .IR operation .TP .BR ESRCH Another thread caused a failure during thread sync, but its ID could not be determined. .SH VERSIONS The .BR seccomp() system call first appeared in Linux 3.17. .\" FIXME Add glibc version .SH CONFORMING TO The .BR seccomp() system call is a nonstandard Linux extension. .SH NOTES .BR seccomp () provides a superset of the functionality provided by the .BR prctl (2) .BR PR_SET_SECCOMP operation (which does not support .IR flags ). .SH EXAMPLE .\" FIXME Please carefully review the following new piece that .\" demonstrates the use of your example program. The program below accepts four or more arguments. The first three arguments are a system call number, a numeric architecture identifier, and an error number. The program uses these values to construct a BPF filter that is used at run time to perform the following checks: .IP [1] 4 If the program is not running on the specified architecture, the BPF filter causes system calls to fail with the error .BR ENOSYS . .IP [2] If the program attempts to execute the system call with the specified number, the BPF filter causes the system call to fail, with .I errno being set to the specified error number. .PP The remaining command-line arguments specify the pathname and additional arguments of a program that the example program should attempt to execute using .BR execve (3) (a library function that employs the .BR execve (2) system call). Some example runs of the program are shown below. First, we display the architecture that we are running on (x86-64) and then construct a shell function that looks up system call numbers on this architecture: .nf .in +4n $ \fBuname -m\fP x86_64 $ \fBsyscall_nr() { cat /usr/src/linux/arch/x86/syscalls/syscall_64.tbl | \\ awk '$2 != "x32" && $3 == "'$1'" { print $1 }' }\fP .in .fi When the BPF filter rejects a system call (case [2] above), it causes the system call to fail with the error number specified on the command line. In the experiments shown here, we'll use error number 99: .nf .in +4n $ \fBerrno 99\fP EADDRNOTAVAIL 99 Cannot assign requested address .in .fi In the following example, we attempt to run the command .BR whoami (1), but the BPF filter rejects the .BR execve (2) system call, so that the command is not even executed: .nf .in +4n $ \fBsyscall_nr execve\fP 59 $ \fB./a.out 59 0xC000003E 99 /bin/whoami\fP execv: Cannot assign requested address .in .fi In the next example, the BPF filter rejects the .BR write (2) system call, so that, although it is successfully started, the .BR whoami (1) command is not able to write output: .nf .in +4n $ \fBsyscall_nr write\fP 1 $ \fB./a.out 1 0xC000003E 99 /bin/whoami\fP .in .fi In the final example, the BPF filter rejects a system call that is not used by the .BR whoami (1) command, so it is able to successfully execute and produce output: .nf .in +4n $ \fBsyscall_nr preadv\fP 295 $ \fB./a.out 295 0xC000003E 99 /bin/whoami\fP cecilia .in .fi .SS Program source .fi .nf #include #include #include #include #include #include #include #include #include static int install_filter(int syscall, int arch, int error) { struct sock_filter filter[] = { /* [0] Load architecture */ BPF_STMT(BPF_LD + BPF_W + BPF_ABS, (offsetof(struct seccomp_data, arch))), /* [1] Jump forward 4 instructions on architecture mismatch */ BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, arch, 0, 4), /* [2] Load system call number */ BPF_STMT(BPF_LD + BPF_W + BPF_ABS, (offsetof(struct seccomp_data, nr))), /* [3] Jump forward 1 instruction on system call number mismatch */ BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, syscall, 0, 1), /* [4] Matching architecture and system call: return specific errno */ BPF_STMT(BPF_RET + BPF_K, SECCOMP_RET_ERRNO | (error & SECCOMP_RET_DATA)), /* [5] Destination of system call number mismatch: allow other system calls */ BPF_STMT(BPF_RET + BPF_K, SECCOMP_RET_ALLOW), /* [6] Destination of architecture mismatch: kill process */ BPF_STMT(BPF_RET + BPF_K, SECCOMP_RET_KILL), }; struct sock_fprog prog = { .len = (unsigned short) (sizeof(filter) / sizeof(filter[0])), .filter = filter, }; if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog)) { perror("seccomp"); return 1; } return 0; } int main(int argc, char **argv) { if (argc < 5) { fprintf(stderr, "Usage:\\n" "refuse []\\n" "Hint: AUDIT_ARCH_I386: 0x%X\\n" " AUDIT_ARCH_X86_64: 0x%X\\n" "\\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64); exit(EXIT_FAILURE); } if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) { perror("prctl"); exit(EXIT_FAILURE); } if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0), strtol(argv[3], NULL, 0))) exit(EXIT_FAILURE); execv(argv[4], &argv[4]); perror("execv"); exit(EXIT_FAILURE); } .fi .SH SEE ALSO .BR prctl (2), .BR ptrace (2), .BR signal (7), .BR socket (7) .sp .\" FIXME: Is the following the best source of info on the BPF language? The kernel source file .IR Documentation/networking/filter.txt .