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NAME

       seccomp - operate on Secure Computing state of the process

SYNOPSIS

       #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

       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 task exiting immediately without executing the system
              call.  The task terminates as though killed by a SIGSYS signal (not SIGKILL).  Even
              if  a  signal handler has been registered and otherwise catches SIGSYS, the handler
              will be ignored in this case and the task always terminates.

              Before Linux 4.11, any process terminated this way would  not  trigger  a  coredump
              (even  though  SIGSYS  is  documented  in  signal(7)  as having a default action of
              termination with a core dump).  Since Linux 4.11, single threaded processes  follow
              standard core dump behavior, but multithreaded processes still do not.  There is no
              workaround currently for multithreaded processes.

       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

       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

       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

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

CONFORMING TO

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

NOTES

       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

       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 task */
               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

       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/userspace-api/seccomp_filter.rst  (or Documentation/prctl/seccomp_filter.txt
       before Linux 4.13).

       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

       This page is part of release 4.13 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/.