Use tabs to indent SECCOMP_RET_USER_NOTIF definition, for consistency
with other items in this list.
Signed-off-by: Jakub Wilk <jwilk@jwilk.net>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This patch introduces a means for syscalls matched in seccomp to notify
some other task that a particular filter has been triggered.
The motivation for this is primarily for use with containers. For example,
if a container does an init_module(), we obviously don't want to load this
untrusted code, which may be compiled for the wrong version of the kernel
anyway. Instead, we could parse the module image, figure out which module
the container is trying to load and load it on the host.
As another example, containers cannot mount() in general since various
filesystems assume a trusted image. However, if an orchestrator knows that
e.g. a particular block device has not been exposed to a container for
writing, it want to allow the container to mount that block device (that
is, handle the mount for it).
This patch adds functionality that is already possible via at least two
other means that I know about, both of which involve ptrace(): first, one
could ptrace attach, and then iterate through syscalls via PTRACE_SYSCALL.
Unfortunately this is slow, so a faster version would be to install a
filter that does SECCOMP_RET_TRACE, which triggers a PTRACE_EVENT_SECCOMP.
Since ptrace allows only one tracer, if the container runtime is that
tracer, users inside the container (or outside) trying to debug it will not
be able to use ptrace, which is annoying. It also means that older
distributions based on Upstart cannot boot inside containers using ptrace,
since upstart itself uses ptrace to monitor services while starting.
The actual implementation of this is fairly small, although getting the
synchronization right was/is slightly complex.
Finally, it's worth noting that the classic seccomp TOCTOU of reading
memory data from the task still applies here, but can be avoided with
careful design of the userspace handler: if the userspace handler reads all
of the task memory that is necessary before applying its security policy,
the tracee's subsequent memory edits will not be read by the tracer.
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
CC: Kees Cook <keescook@chromium.org>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Eric W. Biederman <ebiederm@xmission.com>
CC: "Serge E. Hallyn" <serge@hallyn.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
CC: Christian Brauner <christian@brauner.io>
CC: Tyler Hicks <tyhicks@canonical.com>
CC: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
Signed-off-by: Kees Cook <keescook@chromium.org>
Seccomp logging for "handled" actions such as RET_TRAP, RET_TRACE, or
RET_ERRNO can be very noisy for processes that are being audited. This
patch modifies the seccomp logging behavior to treat processes that are
being inspected via the audit subsystem the same as processes that
aren't under inspection. Handled actions will no longer be logged just
because the process is being inspected. Since v4.14, applications have
the ability to request logging of handled actions by using the
SECCOMP_FILTER_FLAG_LOG flag when loading seccomp filters.
With this patch, the logic for deciding if an action will be logged is:
if action == RET_ALLOW:
do not log
else if action not in actions_logged:
do not log
else if action == RET_KILL:
log
else if action == RET_LOG:
log
else if filter-requests-logging:
log
else:
do not log
Reported-by: Steve Grubb <sgrubb@redhat.com>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Right now, SECCOMP_RET_KILL_THREAD (neé SECCOMP_RET_KILL) kills the
current thread. There have been a few requests for this to kill the entire
process (the thread group). This cannot be just changed (discovered when
adding coredump support since coredumping kills the entire process)
because there are userspace programs depending on the thread-kill
behavior.
Instead, implement SECCOMP_RET_KILL_PROCESS, which is 0x80000000, and can
be processed as "-1" by the kernel, below the existing RET_KILL that is
ABI-set to "0". For userspace, SECCOMP_RET_ACTION_FULL is added to expand
the mask to the signed bit. Old userspace using the SECCOMP_RET_ACTION
mask will see SECCOMP_RET_KILL_PROCESS as 0 still, but this would only
be visible when examining the siginfo in a core dump from a RET_KILL_*,
where it will think it was thread-killed instead of process-killed.
Attempts to introduce this behavior via other ways (filter flags,
seccomp struct flags, masked RET_DATA bits) all come with weird
side-effects and baggage. This change preserves the central behavioral
expectations of the seccomp filter engine without putting too great
a burden on changes needed in userspace to use the new action.
The new action is discoverable by userspace through either the new
actions_avail sysctl or through the SECCOMP_GET_ACTION_AVAIL seccomp
operation. If used without checking for availability, old kernels
will treat RET_KILL_PROCESS as RET_KILL_THREAD (since the old mask
will produce RET_KILL_THREAD).
Cc: Paul Moore <paul@paul-moore.com>
Cc: Fabricio Voznika <fvoznika@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
In preparation for adding SECCOMP_RET_KILL_PROCESS, rename SECCOMP_RET_KILL
to the more accurate SECCOMP_RET_KILL_THREAD.
The existing selftest values are intentionally left as SECCOMP_RET_KILL
just to be sure we're exercising the alias.
Signed-off-by: Kees Cook <keescook@chromium.org>
Add a new action, SECCOMP_RET_LOG, that logs a syscall before allowing
the syscall. At the implementation level, this action is identical to
the existing SECCOMP_RET_ALLOW action. However, it can be very useful when
initially developing a seccomp filter for an application. The developer
can set the default action to be SECCOMP_RET_LOG, maybe mark any
obviously needed syscalls with SECCOMP_RET_ALLOW, and then put the
application through its paces. A list of syscalls that triggered the
default action (SECCOMP_RET_LOG) can be easily gleaned from the logs and
that list can be used to build the syscall whitelist. Finally, the
developer can change the default action to the desired value.
This provides a more friendly experience than seeing the application get
killed, then updating the filter and rebuilding the app, seeing the
application get killed due to a different syscall, then updating the
filter and rebuilding the app, etc.
The functionality is similar to what's supported by the various LSMs.
SELinux has permissive mode, AppArmor has complain mode, SMACK has
bring-up mode, etc.
SECCOMP_RET_LOG is given a lower value than SECCOMP_RET_ALLOW as allow
while logging is slightly more restrictive than quietly allowing.
Unfortunately, the tests added for SECCOMP_RET_LOG are not capable of
inspecting the audit log to verify that the syscall was logged.
With this patch, the logic for deciding if an action will be logged is:
if action == RET_ALLOW:
do not log
else if action == RET_KILL && RET_KILL in actions_logged:
log
else if action == RET_LOG && RET_LOG in actions_logged:
log
else if filter-requests-logging && action in actions_logged:
log
else if audit_enabled && process-is-being-audited:
log
else:
do not log
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Adminstrators can write to this sysctl to set the seccomp actions that
are allowed to be logged. Any actions not found in this sysctl will not
be logged.
For example, all SECCOMP_RET_KILL, SECCOMP_RET_TRAP, and
SECCOMP_RET_ERRNO actions would be loggable if "kill trap errno" were
written to the sysctl. SECCOMP_RET_TRACE actions would not be logged
since its string representation ("trace") wasn't present in the sysctl
value.
The path to the sysctl is:
/proc/sys/kernel/seccomp/actions_logged
The actions_avail sysctl can be read to discover the valid action names
that can be written to the actions_logged sysctl with the exception of
"allow". SECCOMP_RET_ALLOW actions cannot be configured for logging.
The default setting for the sysctl is to allow all actions to be logged
except SECCOMP_RET_ALLOW. While only SECCOMP_RET_KILL actions are
currently logged, an upcoming patch will allow applications to request
additional actions to be logged.
There's one important exception to this sysctl. If a task is
specifically being audited, meaning that an audit context has been
allocated for the task, seccomp will log all actions other than
SECCOMP_RET_ALLOW despite the value of actions_logged. This exception
preserves the existing auditing behavior of tasks with an allocated
audit context.
With this patch, the logic for deciding if an action will be logged is:
if action == RET_ALLOW:
do not log
else if action == RET_KILL && RET_KILL in actions_logged:
log
else if audit_enabled && task-is-being-audited:
log
else:
do not log
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
This patch creates a read-only sysctl containing an ordered list of
seccomp actions that the kernel supports. The ordering, from left to
right, is the lowest action value (kill) to the highest action value
(allow). Currently, a read of the sysctl file would return "kill trap
errno trace allow". The contents of this sysctl file can be useful for
userspace code as well as the system administrator.
The path to the sysctl is:
/proc/sys/kernel/seccomp/actions_avail
libseccomp and other userspace code can easily determine which actions
the current kernel supports. The set of actions supported by the current
kernel may be different than the set of action macros found in kernel
headers that were installed where the userspace code was built.
In addition, this sysctl will allow system administrators to know which
actions are supported by the kernel and make it easier to configure
exactly what seccomp logs through the audit subsystem. Support for this
level of logging configuration will come in a future patch.
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
This updates seccomp_filter.txt for ReST markup, and moves it under the
user-space API index, since it describes how application author can use
seccomp.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Jakub Wilk