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

       seccomp - operate on Secure Computing state of the process

       #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);

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

       Currently, Linux supports the following operation values:

	      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 sig-
	      procmask(2), it can use sigreturn(2) to block all signals	 apart
	      from  SIGKILL  and SIGSTOP.  This means that alarm(2) (for exam-
	      ple) 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 setr-
	      limit(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:


	      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 pro-
	      cesses 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 dan-
	      gerous things because it did not actually drop privileges.)

	      If prctl(2) or seccomp() is allowed by the attached filter, fur-
	      ther  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


	      The recognized flags are:

		     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

		     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.

       When adding filters via SECCOMP_SET_MODE_FILTER, args points to a  fil-
       ter 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 ver-
       ify the value of the arch field.

       It is strongly recommended to use a whitelisting approach whenever pos-
       sible  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 alter-
       ing 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-lan-
       guage 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.  (Proba-
       bly,  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 surpris-
       ing 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 con-
       stant  SECCOMP_RET_ACTION)  contain  one	 of the "action" values listed
       below; the least significant 16-bits  (defined  by  the	constant  SEC-
       COMP_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 exe-
       cution 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:

	      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).

	      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

	      *	 si_syscall and si_arch will indicate which  system  call  was

	      *	 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 informa-

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

	      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

	      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.)

	      This value results in the system call being executed.

       On  success,  seccomp()	returns	 0.    On   error,   if	  SECCOMP_FIL-
       TER_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.

       seccomp() can fail for the following reasons:

	      The caller did not have the CAP_SYS_ADMIN capability in its user
	      namespace,  or  had  not	set  no_new_privs  before  using  SEC-

       EFAULT args was not a valid address.

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

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

       EINVAL A secure computing mode has already been set, and operation dif-
	      fers 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 pro-
	      gram pointed to by args was not valid or the length of the  fil-
	      ter  program  was	 zero or exceeded BPF_MAXINSNS (4096) instruc-

       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

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

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

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

       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	 oper-
	  and whose value is the size of the seccomp_data buffer.

       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 speci-
	   fied 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 addi-
       tional 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

       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
	   $ 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
	   $ ./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
	   $ ./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
	   $ ./a.out 295 0xC000003E 99 /bin/whoami

   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 */
			(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 */
			(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' */

	       /* [6] Destination of system call number mismatch: allow other
		      system calls */

	       /* [7] Destination of architecture mismatch: kill process */

	   struct sock_fprog prog = {
	       .len = (unsigned short) (sizeof(filter) / sizeof(filter[0])),
	       .filter = filter,

	   if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog)) {
	       return 1;

	   return 0;

       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);

	   if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {

	   if (install_filter(strtol(argv[1], NULL, 0),
			      strtol(argv[2], NULL, 0),
			      strtol(argv[3], NULL, 0)))

	   execv(argv[4], &argv[4]);

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

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

       The kernel source files Documentation/networking/filter.txt  and	 Docu-

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

       This  page  is  part of release 4.10 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

Linux				  2016-10-08			    SECCOMP(2)