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Subject[PATCH v9 05/13] seccomp_filter: Document what seccomp_filter is and how it works.
Date
Adds a text file covering what CONFIG_SECCOMP_FILTER is, how it is
implemented presently, and what it may be used for. In addition,
the limitations and caveats of the proposed implementation are
included.

v9: rebase on to bccaeafd7c117acee36e90d37c7e05c19be9e7bf
v8: -
v7: Add a caveat around fork behavior and execve
v6: -
v5: -
v4: rewording (courtesy kees.cook@canonical.com)
reflect support for event ids
add a small section on adding per-arch support
v3: a little more cleanup
v2: moved to prctl/
updated for the v2 syntax.
adds a note about compat behavior

Signed-off-by: Will Drewry <wad@chromium.org>
---
Documentation/prctl/seccomp_filter.txt | 189 ++++++++++++++++++++++++++++++++
1 files changed, 189 insertions(+), 0 deletions(-)
create mode 100644 Documentation/prctl/seccomp_filter.txt
diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt
new file mode 100644
index 0000000..a9cddc2
--- /dev/null
+++ b/Documentation/prctl/seccomp_filter.txt
@@ -0,0 +1,189 @@
+ Seccomp filtering
+ =================
+
+Introduction
+------------
+
+A large number of system calls are exposed to every userland process
+with many of them going unused for the entire lifetime of the process.
+As system calls change and mature, bugs are found and eradicated. A
+certain subset of userland applications benefit by having a reduced set
+of available system calls. The resulting set reduces the total kernel
+surface exposed to the application. System call filtering is meant for
+use with those applications.
+
+The implementation currently leverages both the existing seccomp
+infrastructure and the kernel tracing infrastructure. By centralizing
+hooks for attack surface reduction in seccomp, it is possible to assure
+attention to security that is less relevant in normal ftrace scenarios,
+such as time-of-check, time-of-use attacks. However, ftrace provides a
+rich, human-friendly environment for interfacing with system call
+specific arguments. (As such, this requires FTRACE_SYSCALLS for any
+introspective filtering support.)
+
+
+What it isn't
+-------------
+
+System call filtering isn't a sandbox. It provides a clearly defined
+mechanism for minimizing the exposed kernel surface. Beyond that,
+policy for logical behavior and information flow should be managed with
+a combinations of other system hardening techniques and, potentially, a
+LSM of your choosing. Expressive, dynamic filters based on the ftrace
+filter engine provide further options down this path (avoiding
+pathological sizes or selecting which of the multiplexed system calls in
+socketcall() is allowed, for instance) which could be construed,
+incorrectly, as a more complete sandboxing solution.
+
+
+Usage
+-----
+
+An additional seccomp mode is exposed through mode '2'.
+This mode depends on CONFIG_SECCOMP_FILTER. By default, it provides
+only the most trivial of filter support "1" or cleared. However, if
+CONFIG_FTRACE_SYSCALLS is enabled, the ftrace filter engine may be used
+for more expressive filters.
+
+A collection of filters may be supplied via prctl, and the current set
+of filters is exposed in /proc/<pid>/seccomp_filter.
+
+Interacting with seccomp filters can be done through three new prctl calls
+and one existing one.
+
+PR_SET_SECCOMP:
+ A pre-existing option for enabling strict seccomp mode (1) or
+ filtering seccomp (2).
+
+ Usage:
+ prctl(PR_SET_SECCOMP, 1); /* strict */
+ prctl(PR_SET_SECCOMP, 2); /* filters */
+
+PR_SET_SECCOMP_FILTER:
+ Allows the specification of a new filter for a given system
+ call, by number, and filter string. By default, the filter
+ string may only be "1". However, if CONFIG_FTRACE_SYSCALLS is
+ supported, the filter string may make use of the ftrace
+ filtering language's awareness of system call arguments.
+
+ In addition, the event id for the system call entry may be
+ specified in lieu of the system call number itself, as
+ determined by the 'type' argument. This allows for the future
+ addition of seccomp-based filtering on other registered,
+ relevant ftrace events.
+
+ All calls to PR_SET_SECCOMP_FILTER for a given system
+ call will append the supplied string to any existing filters.
+ Filter construction looks as follows:
+ (Nothing) + "fd == 1 || fd == 2" => fd == 1 || fd == 2
+ ... + "fd != 2" => (fd == 1 || fd == 2) && fd != 2
+ ... + "size < 100" =>
+ ((fd == 1 || fd == 2) && fd != 2) && size < 100
+ If there is no filter and the seccomp mode has already
+ transitioned to filtering, additions cannot be made. Filters
+ may only be added that reduce the available kernel surface.
+
+ Usage (per the construction example above):
+ unsigned long type = PR_SECCOMP_FILTER_SYSCALL;
+ prctl(PR_SET_SECCOMP_FILTER, type, __NR_write,
+ "fd == 1 || fd == 2");
+ prctl(PR_SET_SECCOMP_FILTER, type, __NR_write,
+ "fd != 2");
+ prctl(PR_SET_SECCOMP_FILTER, type, __NR_write,
+ "size < 100");
+
+ The 'type' argument may be one of PR_SECCOMP_FILTER_SYSCALL or
+ PR_SECCOMP_FILTER_EVENT.
+
+PR_CLEAR_SECCOMP_FILTER:
+ Removes all filter entries for a given system call number or
+ event id. When called prior to entering seccomp filtering mode,
+ it allows for new filters to be applied to the same system call.
+ After transition, however, it completely drops access to the
+ call.
+
+ Usage:
+ prctl(PR_CLEAR_SECCOMP_FILTER,
+ PR_SECCOMP_FILTER_SYSCALL, __NR_open);
+
+PR_GET_SECCOMP_FILTER:
+ Returns the aggregated filter string for a system call into a
+ user-supplied buffer of a given length.
+
+ Usage:
+ prctl(PR_GET_SECCOMP_FILTER,
+ PR_SECCOMP_FILTER_SYSCALL, __NR_write, buf,
+ sizeof(buf));
+
+All of the above calls return 0 on success and non-zero on error. If
+CONFIG_FTRACE_SYSCALLS is not supported and a rich-filter was specified,
+the caller may check the errno for -ENOSYS. The same is true if
+specifying an filter by the event id fails to discover any relevant
+event entries.
+
+
+Example
+-------
+
+Assume a process would like to cleanly read and write to stdin/out/err
+as well as access its filters after seccomp enforcement begins. This
+may be done as follows:
+
+ int filter_syscall(int nr, char *buf) {
+ return prctl(PR_SET_SECCOMP_FILTER, PR_SECCOMP_FILTER_SYSCALL,
+ nr, buf);
+ }
+
+ filter_syscall(__NR_read, "fd == 0");
+ filter_syscall(_NR_write, "fd == 1 || fd == 2");
+ filter_syscall(__NR_exit, "1");
+ filter_syscall(__NR_prctl, "1");
+ prctl(PR_SET_SECCOMP, 2);
+
+ /* Do stuff with fdset . . .*/
+
+ /* Drop read access and keep only write access to fd 1. */
+ prctl(PR_CLEAR_SECCOMP_FILTER, PR_SECCOMP_FILTER_SYSCALL, __NR_read);
+ filter_syscall(__NR_write, "fd != 2");
+
+ /* Perform any final processing . . . */
+ syscall(__NR_exit, 0);
+
+
+Caveats
+-------
+
+- Avoid using a filter of "0" to disable a filter. Always favor calling
+ prctl(PR_CLEAR_SECCOMP_FILTER, ...). Otherwise the behavior may vary
+ depending on if CONFIG_FTRACE_SYSCALLS support exists -- though an
+ error will be returned if the support is missing.
+
+- execve is always blocked. seccomp filters may not cross that boundary.
+
+- Filters can be inherited across fork/clone but only when they are
+ active (e.g., PR_SET_SECCOMP has been set to 2), but not prior to use.
+ This stops the parent process from adding filters that may undermine
+ the child process security or create unexpected behavior after an
+ execve.
+
+- Some platforms support a 32-bit userspace with 64-bit kernels. In
+ these cases (CONFIG_COMPAT), system call numbers may not match across
+ 64-bit and 32-bit system calls. When the first PRCTL_SET_SECCOMP_FILTER
+ is called, the in-memory filters state is annotated with whether the
+ call has been made via the compat interface. All subsequent calls will
+ be checked for compat call mismatch. In the long run, it may make sense
+ to store compat and non-compat filters separately, but that is not
+ supported at present. Once one type of system call interface has been
+ used, it must be continued to be used.
+
+
+Adding architecture support
+-----------------------
+
+Any platform with seccomp support should be able to support the bare
+minimum of seccomp filter features. However, since seccomp_filter
+requires that execve be blocked, it expects the architecture to expose a
+__NR_seccomp_execve define that maps to the execve system call number.
+On platforms where CONFIG_COMPAT applies, __NR_seccomp_execve_32 must
+also be provided. Once those macros exist, "select HAVE_SECCOMP_FILTER"
+support may be added to the architectures Kconfig.
--
1.7.0.4


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