Provided by: manpages-dev_4.04-2_all bug

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, 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(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
       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.

              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—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, 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).

   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 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

       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

       This  page  is  part of release 4.04 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
       http://www.kernel.org/doc/man-pages/.