.\" Copyright (C) 2014 Kees Cook .\" and Copyright (C) 2012 Will Drewry .\" and Copyright (C) 2008 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 .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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 .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. 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 .IR "struct\ sock_fprog" ; it can be designed to filter arbitrary system calls and system call arguments. If the filter is invalid, the call will fail, returning .BR EACCESS in .IR errno . If .BR fork (2), .BR clone (2), or .BR execve (2) are allowed by the filter, any child processes will be constrained to the same filters and system calls as the parent. Prior to using this operation, the process must call .IR "prctl(PR_SET_NO_NEW_PRIVS,\ 1)" or run with .BR CAP_SYS_ADMIN privileges in its namespace. If these are not true, the call will fail and return .BR EACCES in .IR errno . This requirement ensures that filter programs cannot be applied to child processes with greater privileges than the process that installed them. Additionally, if .BR prctl (2) or .BR seccomp (2) is allowed by the attached filter, additional filters may be layered on which will increase evaluation time, but allow 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 current process to the same seccomp filter tree. If any thread cannot do this, 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 syscall 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 A seccomp filter may return any of the following values. If multiple filters exist, the return value for the evaluation of a given system call will always use the highest precedent value. (For example, .BR SECCOMP_RET_KILL will always take precedence.) In precedence order, they are: .TP .BR SECCOMP_RET_KILL Results in the task exiting immediately without executing the system call. The exit status of the task (status & 0x7f) will be .BR SIGSYS , 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 syscall was attempted. The program counter will be as though the system call happened (i.e., it will not point to the syscall 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.) The .BR SECCOMP_RET_DATA portion of the return value will be passed as .IR si_errno . .BR SIGSYS triggered by seccomp will have a .IR si_code of .BR SYS_SECCOMP . .TP .BR SECCOMP_RET_ERRNO Results in the lower 16-bits of the return value being passed to userland 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 ptrace()-based tracer prior to executing the system call. If there is no tracer present, .BR ENOSYS is returned to userland and the system call is not executed. 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 syscall 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 ptrace\(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. If multiple filters exist, the return value for the evaluation of a given system call will always use the highest precedent value. Precedence is only determined using 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 This system call first appeared in Linux 3.17. .\" FIXME Add glibc version .SH CONFORMING TO This system call is a nonstandard Linux extension. .SH NOTES .BR seccomp () provides a superset of the functionality provided by the .BR prctl (2) .IR PR_SET_SECCOMP operation (which does not support .IR flags ). .SH EXAMPLE .nf #include #include #include #include #include #include #include #include #include static int install_filter(int syscall, int arch, int error) { struct sock_filter filter[] = { /* Load architecture. */ BPF_STMT(BPF_LD+BPF_W+BPF_ABS, (offsetof(struct seccomp_data, arch))), /* Jump forward 4 instructions on architecture mismatch. */ BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, arch, 0, 4), /* Load syscall number. */ BPF_STMT(BPF_LD+BPF_W+BPF_ABS, (offsetof(struct seccomp_data, nr))), /* Jump forward 1 instruction on syscall mismatch. */ BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, syscall, 0, 1), /* Matching arch and syscall: return specific errno. */ BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO|(error & SECCOMP_RET_DATA)), /* Destination of syscall mismatch: Allow other syscalls. */ BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), /* Destination of arch 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 EXIT_FAILURE; } return EXIT_SUCCESS; } 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); return EXIT_FAILURE; } if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) { perror("prctl"); return EXIT_FAILURE; } if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0), strtol(argv[3], NULL, 0))) return EXIT_FAILURE; execv(argv[4], &argv[4]); perror("execv"); return EXIT_FAILURE; } .fi .SH SEE ALSO .BR prctl (2), .BR ptrace (2), .BR signal (7), .BR socket (7)