Documentation/userspace-api/seccomp_filter.rst

   1 ===========================================
   2 Seccomp BPF (SECure COMPuting with filters)
   3 ===========================================
   4
   5 Introduction
   6 ============
   7
   8 A large number of system calls are exposed to every userland process
   9 with many of them going unused for the entire lifetime of the process.
  10 As system calls change and mature, bugs are found and eradicated.  A
  11 certain subset of userland applications benefit by having a reduced set
  12 of available system calls.  The resulting set reduces the total kernel
  13 surface exposed to the application.  System call filtering is meant for
  14 use with those applications.
  15
  16 Seccomp filtering provides a means for a process to specify a filter for
  17 incoming system calls.  The filter is expressed as a Berkeley Packet
  18 Filter (BPF) program, as with socket filters, except that the data
  19 operated on is related to the system call being made: system call
  20 number and the system call arguments.  This allows for expressive
  21 filtering of system calls using a filter program language with a long
  22 history of being exposed to userland and a straightforward data set.
  23
  24 Additionally, BPF makes it impossible for users of seccomp to fall prey
  25 to time-of-check-time-of-use (TOCTOU) attacks that are common in system
  26 call interposition frameworks.  BPF programs may not dereference
  27 pointers which constrains all filters to solely evaluating the system
  28 call arguments directly.
  29
  30 What it isn't
  31 =============
  32
  33 System call filtering isn't a sandbox.  It provides a clearly defined
  34 mechanism for minimizing the exposed kernel surface.  It is meant to be
  35 a tool for sandbox developers to use.  Beyond that, policy for logical
  36 behavior and information flow should be managed with a combination of
  37 other system hardening techniques and, potentially, an LSM of your
  38 choosing.  Expressive, dynamic filters provide further options down this
  39 path (avoiding pathological sizes or selecting which of the multiplexed
  40 system calls in socketcall() is allowed, for instance) which could be
  41 construed, incorrectly, as a more complete sandboxing solution.
  42
  43 Usage
  44 =====
  45
  46 An additional seccomp mode is added and is enabled using the same
  47 prctl(2) call as the strict seccomp.  If the architecture has
  48 ``CONFIG_HAVE_ARCH_SECCOMP_FILTER``, then filters may be added as below:
  49
  50 ``PR_SET_SECCOMP``:
  51         Now takes an additional argument which specifies a new filter
  52         using a BPF program.
  53         The BPF program will be executed over struct seccomp_data
  54         reflecting the system call number, arguments, and other
  55         metadata.  The BPF program must then return one of the
  56         acceptable values to inform the kernel which action should be
  57         taken.
  58
  59         Usage::
  60
  61                 prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog);
  62
  63         The 'prog' argument is a pointer to a struct sock_fprog which
  64         will contain the filter program.  If the program is invalid, the
  65         call will return -1 and set errno to ``EINVAL``.
  66
  67         If ``fork``/``clone`` and ``execve`` are allowed by @prog, any child
  68         processes will be constrained to the same filters and system
  69         call ABI as the parent.
  70
  71         Prior to use, the task must call ``prctl(PR_SET_NO_NEW_PRIVS, 1)`` or
  72         run with ``CAP_SYS_ADMIN`` privileges in its namespace.  If these are not
  73         true, ``-EACCES`` will be returned.  This requirement ensures that filter
  74         programs cannot be applied to child processes with greater privileges
  75         than the task that installed them.
  76
  77         Additionally, if ``prctl(2)`` is allowed by the attached filter,
  78         additional filters may be layered on which will increase evaluation
  79         time, but allow for further decreasing the attack surface during
  80         execution of a process.
  81
  82 The above call returns 0 on success and non-zero on error.
  83
  84 Return values
  85 =============
  86
  87 A seccomp filter may return any of the following values. If multiple
  88 filters exist, the return value for the evaluation of a given system
  89 call will always use the highest precedent value. (For example,
  90 ``SECCOMP_RET_KILL_PROCESS`` will always take precedence.)
  91
  92 In precedence order, they are:
  93
  94 ``SECCOMP_RET_KILL_PROCESS``:
  95         Results in the entire process exiting immediately without executing
  96         the system call.  The exit status of the task (``status & 0x7f``)
  97         will be ``SIGSYS``, not ``SIGKILL``.
  98
  99 ``SECCOMP_RET_KILL_THREAD``:
 100         Results in the task exiting immediately without executing the
 101         system call.  The exit status of the task (``status & 0x7f``) will
 102         be ``SIGSYS``, not ``SIGKILL``.
 103
 104 ``SECCOMP_RET_TRAP``:
 105         Results in the kernel sending a ``SIGSYS`` signal to the triggering
 106         task without executing the system call. ``siginfo->si_call_addr``
 107         will show the address of the system call instruction, and
 108         ``siginfo->si_syscall`` and ``siginfo->si_arch`` will indicate which
 109         syscall was attempted.  The program counter will be as though
 110         the syscall happened (i.e. it will not point to the syscall
 111         instruction).  The return value register will contain an arch-
 112         dependent value -- if resuming execution, set it to something
 113         sensible.  (The architecture dependency is because replacing
 114         it with ``-ENOSYS`` could overwrite some useful information.)
 115
 116         The ``SECCOMP_RET_DATA`` portion of the return value will be passed
 117         as ``si_errno``.
 118
 119         ``SIGSYS`` triggered by seccomp will have a si_code of ``SYS_SECCOMP``.
 120
 121 ``SECCOMP_RET_ERRNO``:
 122         Results in the lower 16-bits of the return value being passed
 123         to userland as the errno without executing the system call.
 124
 125 ``SECCOMP_RET_TRACE``:
 126         When returned, this value will cause the kernel to attempt to
 127         notify a ``ptrace()``-based tracer prior to executing the system
 128         call.  If there is no tracer present, ``-ENOSYS`` is returned to
 129         userland and the system call is not executed.
 130
 131         A tracer will be notified if it requests ``PTRACE_O_TRACESECCOM``P
 132         using ``ptrace(PTRACE_SETOPTIONS)``.  The tracer will be notified
 133         of a ``PTRACE_EVENT_SECCOMP`` and the ``SECCOMP_RET_DATA`` portion of
 134         the BPF program return value will be available to the tracer
 135         via ``PTRACE_GETEVENTMSG``.
 136
 137         The tracer can skip the system call by changing the syscall number
 138         to -1.  Alternatively, the tracer can change the system call
 139         requested by changing the system call to a valid syscall number.  If
 140         the tracer asks to skip the system call, then the system call will
 141         appear to return the value that the tracer puts in the return value
 142         register.
 143
 144         The seccomp check will not be run again after the tracer is
 145         notified.  (This means that seccomp-based sandboxes MUST NOT
 146         allow use of ptrace, even of other sandboxed processes, without
 147         extreme care; ptracers can use this mechanism to escape.)
 148
 149 ``SECCOMP_RET_LOG``:
 150         Results in the system call being executed after it is logged. This
 151         should be used by application developers to learn which syscalls their
 152         application needs without having to iterate through multiple test and
 153         development cycles to build the list.
 154
 155         This action will only be logged if "log" is present in the
 156         actions_logged sysctl string.
 157
 158 ``SECCOMP_RET_ALLOW``:
 159         Results in the system call being executed.
 160
 161 If multiple filters exist, the return value for the evaluation of a
 162 given system call will always use the highest precedent value.
 163
 164 Precedence is only determined using the ``SECCOMP_RET_ACTION`` mask.  When
 165 multiple filters return values of the same precedence, only the
 166 ``SECCOMP_RET_DATA`` from the most recently installed filter will be
 167 returned.
 168
 169 Pitfalls
 170 ========
 171
 172 The biggest pitfall to avoid during use is filtering on system call
 173 number without checking the architecture value.  Why?  On any
 174 architecture that supports multiple system call invocation conventions,
 175 the system call numbers may vary based on the specific invocation.  If
 176 the numbers in the different calling conventions overlap, then checks in
 177 the filters may be abused.  Always check the arch value!
 178
 179 Example
 180 =======
 181
 182 The ``samples/seccomp/`` directory contains both an x86-specific example
 183 and a more generic example of a higher level macro interface for BPF
 184 program generation.
 185
 186 Sysctls
 187 =======
 188
 189 Seccomp's sysctl files can be found in the ``/proc/sys/kernel/seccomp/``
 190 directory. Here's a description of each file in that directory:
 191
 192 ``actions_avail``:
 193         A read-only ordered list of seccomp return values (refer to the
 194         ``SECCOMP_RET_*`` macros above) in string form. The ordering, from
 195         left-to-right, is the least permissive return value to the most
 196         permissive return value.
 197
 198         The list represents the set of seccomp return values supported
 199         by the kernel. A userspace program may use this list to
 200         determine if the actions found in the ``seccomp.h``, when the
 201         program was built, differs from the set of actions actually
 202         supported in the current running kernel.
 203
 204 ``actions_logged``:
 205         A read-write ordered list of seccomp return values (refer to the
 206         ``SECCOMP_RET_*`` macros above) that are allowed to be logged. Writes
 207         to the file do not need to be in ordered form but reads from the file
 208         will be ordered in the same way as the actions_avail sysctl.
 209
 210         It is important to note that the value of ``actions_logged`` does not
 211         prevent certain actions from being logged when the audit subsystem is
 212         configured to audit a task. If the action is not found in
 213         ``actions_logged`` list, the final decision on whether to audit the
 214         action for that task is ultimately left up to the audit subsystem to
 215         decide for all seccomp return values other than ``SECCOMP_RET_ALLOW``.
 216
 217         The ``allow`` string is not accepted in the ``actions_logged`` sysctl
 218         as it is not possible to log ``SECCOMP_RET_ALLOW`` actions. Attempting
 219         to write ``allow`` to the sysctl will result in an EINVAL being
 220         returned.
 221
 222 Adding architecture support
 223 ===========================
 224
 225 See ``arch/Kconfig`` for the authoritative requirements.  In general, if an
 226 architecture supports both ptrace_event and seccomp, it will be able to
 227 support seccomp filter with minor fixup: ``SIGSYS`` support and seccomp return
 228 value checking.  Then it must just add ``CONFIG_HAVE_ARCH_SECCOMP_FILTER``
 229 to its arch-specific Kconfig.
 230
 231
 232
 233 Caveats
 234 =======
 235
 236 The vDSO can cause some system calls to run entirely in userspace,
 237 leading to surprises when you run programs on different machines that
 238 fall back to real syscalls.  To minimize these surprises on x86, make
 239 sure you test with
 240 ``/sys/devices/system/clocksource/clocksource0/current_clocksource`` set to
 241 something like ``acpi_pm``.
 242
 243 On x86-64, vsyscall emulation is enabled by default.  (vsyscalls are
 244 legacy variants on vDSO calls.)  Currently, emulated vsyscalls will
 245 honor seccomp, with a few oddities:
 246
 247 - A return value of ``SECCOMP_RET_TRAP`` will set a ``si_call_addr`` pointing to
 248   the vsyscall entry for the given call and not the address after the
 249   'syscall' instruction.  Any code which wants to restart the call
 250   should be aware that (a) a ret instruction has been emulated and (b)
 251   trying to resume the syscall will again trigger the standard vsyscall
 252   emulation security checks, making resuming the syscall mostly
 253   pointless.
 254
 255 - A return value of ``SECCOMP_RET_TRACE`` will signal the tracer as usual,
 256   but the syscall may not be changed to another system call using the
 257   orig_rax register. It may only be changed to -1 order to skip the
 258   currently emulated call. Any other change MAY terminate the process.
 259   The rip value seen by the tracer will be the syscall entry address;
 260   this is different from normal behavior.  The tracer MUST NOT modify
 261   rip or rsp.  (Do not rely on other changes terminating the process.
 262   They might work.  For example, on some kernels, choosing a syscall
 263   that only exists in future kernels will be correctly emulated (by
 264   returning ``-ENOSYS``).
 265
 266 To detect this quirky behavior, check for ``addr & ~0x0C00 ==
 267 0xFFFFFFFFFF600000``.  (For ``SECCOMP_RET_TRACE``, use rip.  For
 268 ``SECCOMP_RET_TRAP``, use ``siginfo->si_call_addr``.)  Do not check any other
 269 condition: future kernels may improve vsyscall emulation and current
 270 kernels in vsyscall=native mode will behave differently, but the
 271 instructions at ``0xF...F600{0,4,8,C}00`` will not be system calls in these
 272 cases.
 273
 274 Note that modern systems are unlikely to use vsyscalls at all -- they
 275 are a legacy feature and they are considerably slower than standard
 276 syscalls.  New code will use the vDSO, and vDSO-issued system calls
 277 are indistinguishable from normal system calls.