NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

SECCOMP(2)                Linux Programmer's Manual               SECCOMP(2)

NAME         top

       seccomp - operate on Secure Computing state of the process

SYNOPSIS         top

       #include <linux/seccomp.h>
       #include <linux/filter.h>
       #include <linux/audit.h>
       #include <linux/signal.h>
       #include <sys/ptrace.h>

       int seccomp(unsigned int operation, unsigned int flags, void *args);

DESCRIPTION         top

       The seccomp() system call operates on the Secure Computing (seccomp)
       state of the calling process.

       Currently, Linux supports the following operation values:

       SECCOMP_SET_MODE_STRICT
              The only system calls that the calling thread is permitted to
              make are read(2), write(2), _exit(2) (but not exit_group(2)),
              and sigreturn(2).  Other system calls result in the delivery
              of a 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.

              Note that although the calling thread can no longer call
              sigprocmask(2), it can use sigreturn(2) to block all signals
              apart from SIGKILL and SIGSTOP.  This means that alarm(2) (for
              example) is not sufficient for restricting the process's
              execution time.  Instead, to reliably terminate the process,
              SIGKILL must be used.  This can be done by using
              timer_create(2) with SIGEV_SIGNAL and sigev_signo set to
              SIGKILL, or by using setrlimit(2) to set the hard limit for
              RLIMIT_CPU.

              This operation is available only if the kernel is configured
              with CONFIG_SECCOMP enabled.

              The value of flags must be 0, and args must be NULL.

              This operation is functionally identical to the call:

                  prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);

       SECCOMP_SET_MODE_FILTER
              The system calls allowed are defined by a pointer to a
              Berkeley Packet Filter (BPF) passed via args.  This argument
              is a pointer to a struct sock_fprog; it can be designed to
              filter arbitrary system calls and system call arguments.  If
              the filter is invalid, seccomp() fails, returning EINVAL in
              errno.

              If fork(2) or clone(2) is allowed by the filter, any child
              processes will be constrained to the same system call filters
              as the parent.  If execve(2) is allowed, the existing filters
              will be preserved across a call to execve(2).

              In order to use the SECCOMP_SET_MODE_FILTER operation, either
              the caller must have the CAP_SYS_ADMIN capability in its user
              namespace, or the thread must already have the no_new_privs
              bit set.  If that bit was not already set by an ancestor of
              this thread, the thread must make the following call:

                  prctl(PR_SET_NO_NEW_PRIVS, 1);

              Otherwise, the SECCOMP_SET_MODE_FILTER operation will fail and
              return EACCES in 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
              execve(2), thus potentially compromising that program.  (Such
              a malicious filter might, for example, cause an attempt to use
              setuid(2) to set the caller's user IDs to non-zero values to
              instead return 0 without actually making the system call.
              Thus, the program might be tricked into retaining superuser
              privileges in circumstances where it is possible to influence
              it to do dangerous things because it did not actually drop
              privileges.)

              If prctl(2) or seccomp() 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 thread.

              The SECCOMP_SET_MODE_FILTER operation is available only if the
              kernel is configured with CONFIG_SECCOMP_FILTER enabled.

              When flags is 0, this operation is functionally identical to
              the call:

                  prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, args);

              The recognized flags are:

              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
                     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 in the same process is in
                     SECCOMP_MODE_STRICT or if it has attached new seccomp
                     filters to itself, diverging from the calling thread's
                     filter tree.

   Filters
       When adding filters via SECCOMP_SET_MODE_FILTER, args points to a
       filter program:

           struct sock_fprog {
               unsigned short      len;    /* Number of BPF instructions */
               struct sock_filter *filter; /* Pointer to array of
                                              BPF instructions */
           };

       Each program must contain one or more BPF instructions:

           struct sock_filter {            /* Filter block */
               __u16 code;                 /* Actual filter code */
               __u8  jt;                   /* Jump true */
               __u8  jf;                   /* Jump false */
               __u32 k;                    /* Generic multiuse field */
           };

       When executing the instructions, the BPF program operates on the
       system call information made available (i.e., use the BPF_ABS
       addressing mode) as a (read-only) buffer of the following form:

           struct seccomp_data {
               int   nr;                   /* System call number */
               __u32 arch;                 /* AUDIT_ARCH_* value
                                              (see <linux/audit.h>) */
               __u64 instruction_pointer;  /* CPU instruction pointer */
               __u64 args[6];              /* Up to 6 system call arguments */
           };

       Because numbering of system calls varies between architectures and
       some architectures (e.g., x86-64) allow user-space code to use the
       calling conventions of multiple architectures, it is usually
       necessary to verify the value of the arch field.

       It is strongly recommended to use a whitelisting approach whenever
       possible because such an approach is more robust and simple.  A
       blacklist will have to be updated whenever a potentially dangerous
       system call is added (or a dangerous flag or option if those are
       blacklisted), and it is often possible to alter the representation of
       a value without altering its meaning, leading to a blacklist bypass.

       The arch field is not unique for all calling conventions.  The x86-64
       ABI and the x32 ABI both use AUDIT_ARCH_X86_64 as arch, and they run
       on the same processors.  Instead, the mask __X32_SYSCALL_BIT is used
       on the system call number to tell the two ABIs apart.

       This means that in order to create a seccomp-based blacklist for
       system calls performed through the x86-64 ABI, it is necessary to not
       only check that arch equals AUDIT_ARCH_X86_64, but also to explicitly
       reject all system calls that contain __X32_SYSCALL_BIT in nr.

       The instruction_pointer field provides the address of the machine-
       language instruction that performed the system call.  This might be
       useful in conjunction with the use of /proc/[pid]/maps to perform
       checks based on which region (mapping) of the program made the system
       call.  (Probably, it is wise to lock down the mmap(2) and mprotect(2)
       system calls to prevent the program from subverting such checks.)

       When checking values from args against a blacklist, keep in mind that
       arguments are often silently truncated before being processed, but
       after the seccomp check.  For example, this happens if the i386 ABI
       is used on an x86-64 kernel: although the kernel will normally not
       look beyond the 32 lowest bits of the arguments, the values of the
       full 64-bit registers will be present in the seccomp data.  A less
       surprising example is that if the x86-64 ABI is used to perform a
       system call that takes an argument of type int, the more-significant
       half of the argument register is ignored by the system call, but
       visible in the seccomp data.

       A seccomp filter returns a 32-bit value consisting of two parts: the
       most significant 16 bits (corresponding to the mask defined by the
       constant SECCOMP_RET_ACTION) contain one of the "action" values
       listed below; the least significant 16-bits (defined by the constant
       SECCOMP_RET_DATA) are "data" to be associated with this return value.

       If multiple filters exist, they are all executed, in reverse order of
       their addition to the filter tree—that is, the most recently
       installed filter is executed first.  (Note that all filters will be
       called even if one of the earlier filters returns SECCOMP_RET_KILL.
       This is done to simplify the kernel code and to provide a tiny speed-
       up in the execution of sets of filters by avoiding a check for this
       uncommon case.)  The return value for the evaluation of a given
       system call is the first-seen SECCOMP_RET_ACTION value of highest
       precedence (along with its accompanying data) returned by execution
       of all of the filters.

       In decreasing order of precedence, the values that may be returned by
       a seccomp filter are:

       SECCOMP_RET_KILL
              This value results in the process exiting immediately without
              executing the system call.  The process terminates as though
              killed by a SIGSYS signal (not SIGKILL).

       SECCOMP_RET_TRAP
              This value results in the kernel sending a SIGSYS signal to
              the triggering process without executing the system call.
              Various fields will be set in the siginfo_t structure (see
              sigaction(2)) associated with signal:

              *  si_signo will contain SIGSYS.

              *  si_call_addr will show the address of the system call
                 instruction.

              *  si_syscall and si_arch will indicate which system call was
                 attempted.

              *  si_code will contain SYS_SECCOMP.

              *  si_errno will contain the SECCOMP_RET_DATA portion of the
                 filter return value.

              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 appropriate
              for the system call.  (The architecture dependency is because
              replacing it with ENOSYS could overwrite some useful
              information.)

       SECCOMP_RET_ERRNO
              This value results in the SECCOMP_RET_DATA portion of the
              filter's return value being passed to user space as the errno
              value without executing the system call.

       SECCOMP_RET_TRACE
              When returned, this value will cause the kernel to attempt to
              notify a ptrace(2)-based tracer prior to executing the system
              call.  If there is no tracer present, the system call is not
              executed and returns a failure status with errno set to
              ENOSYS.

              A tracer will be notified if it requests PTRACE_O_TRACESECCOMP
              using ptrace(PTRACE_SETOPTIONS).  The tracer will be notified
              of a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of
              the filter's return value will be available to the tracer via
              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.

              Before kernel 4.8, the seccomp check will not be run again
              after the tracer is notified.  (This means that, on older
              kernels, seccomp-based sandboxes must not allow use of
              ptrace(2)—even of other sandboxed processes—without extreme
              care; ptracers can use this mechanism to escape from the
              seccomp sandbox.)

       SECCOMP_RET_ALLOW
              This value results in the system call being executed.

RETURN VALUE         top

       On success, seccomp() returns 0.  On error, if
       SECCOMP_FILTER_FLAG_TSYNC was used, the return value is the ID of the
       thread that caused the synchronization failure.  (This ID is a kernel
       thread ID of the type returned by clone(2) and gettid(2).)  On other
       errors, -1 is returned, and errno is set to indicate the cause of the
       error.

ERRORS         top

       seccomp() can fail for the following reasons:

       EACCESS
              The caller did not have the CAP_SYS_ADMIN capability in its
              user namespace, or had not set no_new_privs before using
              SECCOMP_SET_MODE_FILTER.

       EFAULT args was not a valid address.

       EINVAL operation is unknown; or flags are invalid for the given
              operation.

       EINVAL operation included BPF_ABS, but the specified offset was not
              aligned to a 32-bit boundary or exceeded
              sizeof(struct seccomp_data).

       EINVAL A secure computing mode has already been set, and operation
              differs from the existing setting.

       EINVAL operation specified SECCOMP_SET_MODE_FILTER, but the kernel
              was not built with CONFIG_SECCOMP_FILTER enabled.

       EINVAL operation specified SECCOMP_SET_MODE_FILTER, but the filter
              program pointed to by args was not valid or the length of the
              filter program was zero or exceeded BPF_MAXINSNS (4096)
              instructions.

       ENOMEM Out of memory.

       ENOMEM The total length of all filter programs attached to the
              calling thread would exceed MAX_INSNS_PER_PATH (32768)
              instructions.  Note that for the purposes of calculating this
              limit, each already existing filter program incurs an overhead
              penalty of 4 instructions.

       ESRCH  Another thread caused a failure during thread sync, but its ID
              could not be determined.

VERSIONS         top

       The seccomp() system call first appeared in Linux 3.17.

CONFORMING TO         top

       The seccomp() system call is a nonstandard Linux extension.

NOTES         top

       Rather than hand-coding seccomp filters as shown in the example
       below, you may prefer to employ the libseccomp library, which
       provides a front-end for generating seccomp filters.

       The Seccomp field of the /proc/[pid]/status file provides a method of
       viewing the seccomp mode of a process; see proc(5).

       seccomp() provides a superset of the functionality provided by the
       prctl(2) PR_SET_SECCOMP operation (which does not support flags).

       Since Linux 4.4, the prctl(2) PTRACE_SECCOMP_GET_FILTER operation can
       be used to dump a process's seccomp filters.

   Seccomp-specific BPF details
       Note the following BPF details specific to seccomp filters:

       *  The BPF_H and BPF_B size modifiers are not supported: all
          operations must load and store (4-byte) words (BPF_W).

       *  To access the contents of the seccomp_data buffer, use the BPF_ABS
          addressing mode modifier.

       *  The BPF_LEN addressing mode modifier yields an immediate mode
          operand whose value is the size of the seccomp_data buffer.

EXAMPLE         top

       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:

       [1] If the program is not running on the specified architecture, the
           BPF filter causes system calls to fail with the error ENOSYS.

       [2] If the program attempts to execute the system call with the
           specified number, the BPF filter causes the system call to fail,
           with errno being set to the specified error number.

       The remaining command-line arguments specify the pathname and
       additional arguments of a program that the example program should
       attempt to execute using execv(3) (a library function that employs
       the 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:

           $ uname -m
           x86_64
           $ syscall_nr() {
               cat /usr/src/linux/arch/x86/syscalls/syscall_64.tbl | \
               awk '$2 != "x32" && $3 == "'$1'" { print $1 }'
           }

       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:

           $ errno 99
           EADDRNOTAVAIL 99 Cannot assign requested address

       In the following example, we attempt to run the command whoami(1),
       but the BPF filter rejects the execve(2) system call, so that the
       command is not even executed:

           $ syscall_nr execve
           59
           $ ./a.out
           Usage: ./a.out <syscall_nr> <arch> <errno> <prog> [<args>]
           Hint for <arch>: AUDIT_ARCH_I386: 0x40000003
                            AUDIT_ARCH_X86_64: 0xC000003E
           $ ./a.out 59 0xC000003E 99 /bin/whoami
           execv: Cannot assign requested address

       In the next example, the BPF filter rejects the write(2) system call,
       so that, although it is successfully started, the whoami(1) command
       is not able to write output:

           $ syscall_nr write
           1
           $ ./a.out 1 0xC000003E 99 /bin/whoami

       In the final example, the BPF filter rejects a system call that is
       not used by the whoami(1) command, so it is able to successfully
       execute and produce output:

           $ syscall_nr preadv
           295
           $ ./a.out 295 0xC000003E 99 /bin/whoami
           cecilia

   Program source
       #include <errno.h>
       #include <stddef.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <linux/audit.h>
       #include <linux/filter.h>
       #include <linux/seccomp.h>
       #include <sys/prctl.h>

       #define X32_SYSCALL_BIT 0x40000000

       static int
       install_filter(int syscall_nr, int t_arch, int f_errno)
       {
           unsigned int upper_nr_limit = 0xffffffff;

           /* Assume that AUDIT_ARCH_X86_64 means the normal x86-64 ABI */
           if (t_arch == AUDIT_ARCH_X86_64)
               upper_nr_limit = X32_SYSCALL_BIT - 1;

           struct sock_filter filter[] = {
               /* [0] Load architecture from 'seccomp_data' buffer into
                      accumulator */
               BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                        (offsetof(struct seccomp_data, arch))),

               /* [1] Jump forward 5 instructions if architecture does not
                      match 't_arch' */
               BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, t_arch, 0, 5),

               /* [2] Load system call number from 'seccomp_data' buffer into
                      accumulator */
               BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                        (offsetof(struct seccomp_data, nr))),

               /* [3] Check ABI - only needed for x86-64 in blacklist use
                      cases.  Use JGT instead of checking against the bit
                      mask to avoid having to reload the syscall number. */
               BPF_JUMP(BPF_JMP | BPF_JGT | BPF_K, upper_nr_limit, 3, 0),

               /* [4] Jump forward 1 instruction if system call number
                      does not match 'syscall_nr' */
               BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, syscall_nr, 0, 1),

               /* [5] Matching architecture and system call: don't execute
                   the system call, and return 'f_errno' in 'errno' */
               BPF_STMT(BPF_RET | BPF_K,
                        SECCOMP_RET_ERRNO | (f_errno & SECCOMP_RET_DATA)),

               /* [6] Destination of system call number mismatch: allow other
                      system calls */
               BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),

               /* [7] 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: "
                       "%s <syscall_nr> <arch> <errno> <prog> [<args>]\n"
                       "Hint for <arch>: AUDIT_ARCH_I386: 0x%X\n"
                       "                 AUDIT_ARCH_X86_64: 0x%X\n"
                       "\n", argv[0], 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);
       }

SEE ALSO         top

       bpf(2), prctl(2), ptrace(2), sigaction(2), proc(5), signal(7),
       socket(7)

       Various pages from the libseccomp library, including:
       scmp_sys_resolver(1), seccomp_init(3), seccomp_load(3),
       seccomp_rule_add(3), and seccomp_export_bpf(3).

       The kernel source files Documentation/networking/filter.txt and
       Documentation/prctl/seccomp_filter.txt.

       McCanne, S. and Jacobson, V. (1992) The BSD Packet Filter: A New
       Architecture for User-level Packet Capture, Proceedings of the USENIX
       Winter 1993 Conference 
       ⟨http://www.tcpdump.org/papers/bpf-usenix93.pdf

COLOPHON         top

       This page is part of release 4.11 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2016-10-08                       SECCOMP(2)

Pages that refer to this page: bpf(2)prctl(2)ptrace(2)sigaction(2)socketcall(2)syscalls(2)seccomp_attr_set(3)proc(5)capabilities(7)signal(7)