seccomp

       #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 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:

                  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 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, or the thread
              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(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 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  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 */

           };

       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.)
       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 informa-
              tion.)

       SECCOMP_RET_ERRNO
              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.

       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.

              The seccomp check will not be run  again  after  the  tracer  is
              notified.   (This  means  that  seccomp-based sandboxes must not
              allow use of ptrace(2)--even of other sandboxed processes--with-

ERRORS
       seccomp() can fail for the following reasons:

       EACCESS
              The caller did not have the CAP_SYS_ADMIN capability, 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 opera-
              tion.

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

       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-
              tions.

       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
          and 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 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
       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
           $ ./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)),


           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),    sec-
       comp_rule_add(3), and seccomp_export_bpf(3).

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

       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>

COLOPHON
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