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man:seccomp

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

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 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 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  fails   and
            returns  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  nonzero  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
            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.
            SECCOMP_FILTER_FLAG_LOG (since Linux 4.14)
                   All filter return actions except SECCOMP_RET_ALLOW should
                   be  logged.   An  administrator  may override this filter
                   flag by preventing specific actions from being logged via
                   the /proc/sys/kernel/seccomp/actions_logged file.
     SECCOMP_GET_ACTION_AVAIL (since Linux 4.14)
            Test to see if an action is supported by the kernel.  This oper-
            ation is helpful to confirm that the  kernel  knows  of  a  more
            recently  added filter return action since the kernel treats all
            unknown actions as SECCOMP_RET_KILL_PROCESS.
            The value of flags must be 0, and args must be a pointer  to  an
            unsigned 32-bit filter return action.
 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 */ };
     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 (and the convention being used
     may vary over the life of a process  that  uses  execve(2)  to  execute
     binaries  that  employ the different conventions), it is usually neces-
     sary to verify 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.  See also Caveats
     below.
     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_FULL)  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  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  action value of highest precedence (along with its accompa-
     nying data) returned by execution of all of the filters.
     In decreasing order of  precedence,  the  action  values  that  may  be
     returned by a seccomp filter are:
     SECCOMP_RET_KILL_PROCESS (since Linux 4.14)
            This value results in immediate termination of the process, with
            a core dump.  The system call is not executed.  By contrast with
            SECCOMP_RET_KILL_THREAD  below,  all threads in the thread group
            are terminated.  (For a discussion of  thread  groups,  see  the
            description of the CLONE_THREAD flag in clone(2).)
            The  process  terminates  as  though  killed by a SIGSYS signal.
            Even if a signal handler has been  registered  for  SIGSYS,  the
            handler will be ignored in this case and the process always ter-
            minates.  To a parent process that is waiting  on  this  process
            (using  waitpid(2)  or similar), the returned wstatus will indi-
            cate that its child was terminated as though by a SIGSYS signal.
     SECCOMP_RET_KILL_THREAD (or SECCOMP_RET_KILL)
            This  value  results in immediate termination of the thread that
            made the system call.  The system call is not  executed.   Other
            threads in the same thread group will continue to execute.
            The  thread terminates as though killed by a SIGSYS signal.  See
            SECCOMP_RET_KILL_PROCESS above.
            Before Linux 4.11, any process terminated in this way would  not
            trigger  a  coredump  (even  though SIGSYS is documented in sig-
            nal(7) as having a default action of  termination  with  a  core
            dump).   Since  Linux  4.11, a single-threaded process will dump
            core if terminated in this way.
            With the addition of  SECCOMP_RET_KILL_PROCESS  in  Linux  4.14,
            SECCOMP_RET_KILL_THREAD   was   added  as  a  synonym  for  SEC-
            COMP_RET_KILL, in order to  more  clearly  distinguish  the  two
            actions.
     SECCOMP_RET_TRAP
            This  value  results  in  the  kernel  sending a thread-directed
            SIGSYS signal to the triggering thread.  (The system call is not
            executed.)   Various  fields will be set in the siginfo_t struc-
            ture (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., the program counter will not point  to  the  system  call
            instruction).   The return value register will contain an archi-
            tecture-dependent value; if resuming execution, set it to  some-
            thing appropriate for the system call.  (The architecture depen-
            dency 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 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.
            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_LOG (since Linux 4.14)
            This value results in the system call being executed  after  the
            filter  return  action is logged.  An administrator may override
            the  logging  of  this  action  via  the   /proc/sys/kernel/sec-
            comp/actions_logged file.
     SECCOMP_RET_ALLOW
            This value results in the system call being executed.
     If  an  action value other than one of the above is specified, then the
     filter action is  treated  as  either  SECCOMP_RET_KILL_PROCESS  (since
     Linux 4.14) or SECCOMP_RET_KILL_THREAD (in Linux 4.13 and earlier).
 /proc interfaces
     The  files in the directory /proc/sys/kernel/seccomp provide additional
     seccomp information and configuration:
     actions_avail (since Linux 4.14)
            A read-only ordered list of seccomp  filter  return  actions  in
            string form.  The ordering, from left-to-right, is in decreasing
            order of precedence.  The list represents  the  set  of  seccomp
            filter return actions supported by the kernel.
     actions_logged (since Linux 4.14)
            A  read-write ordered list of seccomp filter return actions that
            are allowed to be logged.  Writes to the file do not need to  be
            in  ordered  form but reads from the file will be ordered in the
            same way as the actions_avail file.
            It is important to note that the value  of  actions_logged  does
            not prevent certain filter return actions from being logged when
            the audit subsystem is configured  to  audit  a  task.   If  the
            action  is not found in the actions_logged file, the final deci-
            sion on whether to audit the action for that task is  ultimately
            left  up  to the audit subsystem to decide for all filter return
            actions other than SECCOMP_RET_ALLOW.
            The "allow" string is not accepted in the actions_logged file as
            it is not possible to log SECCOMP_RET_ALLOW actions.  Attempting
            to write "allow" to the file will fail with the error EINVAL.
 Audit logging of seccomp actions
     Since Linux 4.14, the kernel provides the facility to log  the  actions
     returned  by  seccomp  filters  in the audit log.  The kernel makes the
     decision to log an action based on the action type,  whether or not the
     action is present in the actions_logged file, and whether kernel audit-
     ing is enabled (e.g., via the kernel boot option audit=1).   The  rules
     are as follows:
  • If the action is SECCOMP_RET_ALLOW, the action is not logged.
  • Otherwise, if the action is either SECCOMP_RET_KILL_PROCESS or SEC-

COMP_RET_KILL_THREAD, and that action appears in the actions_logged

        file, the action is logged.
  • Otherwise, if the filter has requested logging (the SECCOMP_FIL-

TER_FLAG_LOG flag) and the action appears in the actions_logged

        file, the action is logged.
  • Otherwise, if kernel auditing is enabled and the process is being

audited (autrace(8)), the action is logged.

  • Otherwise, the action is not logged.

RETURN VALUE

     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.

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  SEC-
            COMP_SET_MODE_FILTER.
     EFAULT args was not a valid address.
     EINVAL operation is unknown or is not supported by this kernel  version
            or configuration.
     EINVAL The specified 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 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 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.
     EOPNOTSUPP
            operation  specified  SECCOMP_GET_ACTION_AVAIL,  but  the kernel
            does not support the filter return action specified by args.
     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 ptrace(2) PTRACE_SECCOMP_GET_FILTER operation can
     be used to dump a process's seccomp filters.
 Architecture support for seccomp BPF
     Architecture support for seccomp BPF filtering is available on the fol-
     lowing architectures:
  • x86-64, i386, x32 (since Linux 3.5)
  • ARM (since Linux 3.8)
  • s390 (since Linux 3.8)
  • MIPS (since Linux 3.16)
  • ARM-64 (since Linux 3.19)
  • PowerPC (since Linux 4.3)
  • Tile (since Linux 4.3)
  • PA-RISC (since Linux 4.6)
 Caveats
     There  are various subtleties to consider when applying seccomp filters
     to a program, including the following:
  • Some traditional system calls have user-space implementations in the

vdso(7) on many architectures. Notable examples include clock_get-

        time(2), gettimeofday(2), and time(2).  On such architectures,  sec-
        comp  filtering  for  these system calls will have no effect.  (How-
        ever, there are cases where the  vdso(7)  implementations  may  fall
        back to invoking the true system call, in which case seccomp filters
        would see the system call.)
  • Seccomp filtering is based on system call numbers. However, appli-

cations typically do not directly invoke system calls, but instead

        call wrapper functions in the C library which  in  turn  invoke  the
        system calls.  Consequently, one must be aware of the following:
        o  The glibc wrappers for some traditional system calls may actually
           employ system calls with different  names  in  the  kernel.   For
           example,  the  exit(2)  wrapper  function  actually  employs  the
           exit_group(2) system call, and the fork(2) wrapper function actu-
           ally calls clone(2).
        o  The  behavior of wrapper functions may vary across architectures,
           according to the range of system calls provided on  those  archi-
           tectures.   In  other words, the same wrapper function may invoke
           different system calls on different architectures.
        o  Finally, the behavior of  wrapper  functions  can  change  across
           glibc  versions.  For example, in older versions, the glibc wrap-
           per function for open(2) invoked the  system  call  of  the  same
           name,  but starting in glibc 2.26, the implementation switched to
           calling openat(2) on all architectures.
     The consequence of the above points is that it may be necessary to fil-
     ter  for  a  system  call other than might be expected.  Various manual
     pages in Section  2  provide  helpful  details  about  the  differences
     between  wrapper  functions  and the underlying system calls in subsec-
     tions entitled C library/kernel differences.
     Furthermore, note that the application of seccomp  filters  even  risks
     causing bugs in an application, when the filters cause unexpected fail-
     ures for legitimate operations that the application might need to  per-
     form.   Such bugs may not easily be discovered when testing the seccomp
     filters if the bugs occur in rarely used application code paths.
 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.

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
     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
            (in the x32 ABI, all system calls have bit 30 set in the
            'nr' field, meaning the numbers are >= X32_SYSCALL_BIT) */
         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 BPF_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

     strace(1), bpf(2), prctl(2),  ptrace(2),  sigaction(2),  proc(5),  sig-
     nal(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/userspace-api/seccomp_filter.rst (or Documentation/prctl/sec-
     comp_filter.txt before Linux 4.13).
     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

COLOPHON

     This  page  is  part of release 4.16 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 2018-02-02 SECCOMP(2)

/data/webs/external/dokuwiki/data/pages/man/seccomp.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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