Documentation/prctl/seccomp_filter.txt

   1                 SECure COMPuting with filters
   2                 =============================
   3
   4 Introduction
   5 ------------
   6
   7 A large number of system calls are exposed to every userland process
   8 with many of them going unused for the entire lifetime of the process.
   9 As system calls change and mature, bugs are found and eradicated.  A
  10 certain subset of userland applications benefit by having a reduced set
  11 of available system calls.  The resulting set reduces the total kernel
  12 surface exposed to the application.  System call filtering is meant for
  13 use with those applications.
  14
  15 Seccomp filtering provides a means for a process to specify a filter for
  16 incoming system calls.  The filter is expressed as a Berkeley Packet
  17 Filter (BPF) program, as with socket filters, except that the data
  18 operated on is related to the system call being made: system call
  19 number and the system call arguments.  This allows for expressive
  20 filtering of system calls using a filter program language with a long
  21 history of being exposed to userland and a straightforward data set.
  22
  23 Additionally, BPF makes it impossible for users of seccomp to fall prey
  24 to time-of-check-time-of-use (TOCTOU) attacks that are common in system
  25 call interposition frameworks.  BPF programs may not dereference
  26 pointers which constrains all filters to solely evaluating the system
  27 call arguments directly.
  28
  29 What it isn't
  30 -------------
  31
  32 System call filtering isn't a sandbox.  It provides a clearly defined
  33 mechanism for minimizing the exposed kernel surface.  It is meant to be
  34 a tool for sandbox developers to use.  Beyond that, policy for logical
  35 behavior and information flow should be managed with a combination of
  36 other system hardening techniques and, potentially, an LSM of your
  37 choosing.  Expressive, dynamic filters provide further options down this
  38 path (avoiding pathological sizes or selecting which of the multiplexed
  39 system calls in socketcall() is allowed, for instance) which could be
  40 construed, incorrectly, as a more complete sandboxing solution.
  41
  42 Usage
  43 -----
  44
  45 An additional seccomp mode is added and is enabled using the same
  46 prctl(2) call as the strict seccomp.  If the architecture has
  47 CONFIG_HAVE_ARCH_SECCOMP_FILTER, then filters may be added as below:
  48
  49 PR_SET_SECCOMP:
  50         Now takes an additional argument which specifies a new filter
  51         using a BPF program.
  52         The BPF program will be executed over struct seccomp_data
  53         reflecting the system call number, arguments, and other
  54         metadata.  The BPF program must then return one of the
  55         acceptable values to inform the kernel which action should be
  56         taken.
  57
  58         Usage:
  59                 prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog);
  60
  61         The 'prog' argument is a pointer to a struct sock_fprog which
  62         will contain the filter program.  If the program is invalid, the
  63         call will return -1 and set errno to EINVAL.
  64
  65         If fork/clone and execve are allowed by @prog, any child
  66         processes will be constrained to the same filters and system
  67         call ABI as the parent.
  68
  69         Prior to use, the task must call prctl(PR_SET_NO_NEW_PRIVS, 1) or
  70         run with CAP_SYS_ADMIN privileges in its namespace.  If these are not
  71         true, -EACCES will be returned.  This requirement ensures that filter
  72         programs cannot be applied to child processes with greater privileges
  73         than the task that installed them.
  74
  75         Additionally, if prctl(2) is allowed by the attached filter,
  76         additional filters may be layered on which will increase evaluation
  77         time, but allow for further decreasing the attack surface during
  78         execution of a process.
  79
  80 The above call returns 0 on success and non-zero on error.
  81
  82 Return values
  83 -------------
  84 A seccomp filter may return any of the following values. If multiple
  85 filters exist, the return value for the evaluation of a given system
  86 call will always use the highest precedent value. (For example,
  87 SECCOMP_RET_KILL will always take precedence.)
  88
  89 In precedence order, they are:
  90
  91 SECCOMP_RET_KILL:
  92         Results in the task exiting immediately without executing the
  93         system call.  The exit status of the task (status & 0x7f) will
  94         be SIGSYS, not SIGKILL.
  95
  96 SECCOMP_RET_TRAP:
  97         Results in the kernel sending a SIGSYS signal to the triggering
  98         task without executing the system call.  The kernel will
  99         rollback the register state to just before the system call
 100         entry such that a signal handler in the task will be able to
 101         inspect the ucontext_t->uc_mcontext registers and emulate
 102         system call success or failure upon return from the signal
 103         handler.
 104
 105         The SECCOMP_RET_DATA portion of the return value will be passed
 106         as si_errno.
 107
 108         SIGSYS triggered by seccomp will have a si_code of SYS_SECCOMP.
 109
 110 SECCOMP_RET_ERRNO:
 111         Results in the lower 16-bits of the return value being passed
 112         to userland as the errno without executing the system call.
 113
 114 SECCOMP_RET_TRACE:
 115         When returned, this value will cause the kernel to attempt to
 116         notify a ptrace()-based tracer prior to executing the system
 117         call.  If there is no tracer present, -ENOSYS is returned to
 118         userland and the system call is not executed.
 119
 120         A tracer will be notified if it requests PTRACE_O_TRACESECCOMP
 121         using ptrace(PTRACE_SETOPTIONS).  The tracer will be notified
 122         of a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of
 123         the BPF program return value will be available to the tracer
 124         via PTRACE_GETEVENTMSG.
 125
 126 SECCOMP_RET_ALLOW:
 127         Results in the system call being executed.
 128
 129 If multiple filters exist, the return value for the evaluation of a
 130 given system call will always use the highest precedent value.
 131
 132 Precedence is only determined using the SECCOMP_RET_ACTION mask.  When
 133 multiple filters return values of the same precedence, only the
 134 SECCOMP_RET_DATA from the most recently installed filter will be
 135 returned.
 136
 137 Pitfalls
 138 --------
 139
 140 The biggest pitfall to avoid during use is filtering on system call
 141 number without checking the architecture value.  Why?  On any
 142 architecture that supports multiple system call invocation conventions,
 143 the system call numbers may vary based on the specific invocation.  If
 144 the numbers in the different calling conventions overlap, then checks in
 145 the filters may be abused.  Always check the arch value!
 146
 147 Example
 148 -------
 149
 150 The samples/seccomp/ directory contains both an x86-specific example
 151 and a more generic example of a higher level macro interface for BPF
 152 program generation.
 153
 154
 155
 156 Adding architecture support
 157 -----------------------
 158
 159 See arch/Kconfig for the authoritative requirements.  In general, if an
 160 architecture supports both ptrace_event and seccomp, it will be able to
 161 support seccomp filter with minor fixup: SIGSYS support and seccomp return
 162 value checking.  Then it must just add CONFIG_HAVE_ARCH_SECCOMP_FILTER
 163 to its arch-specific Kconfig.