seccomp: add a return code to trap to userspace

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>

1 files changed, 84 insertions, 0 deletions

diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
index 82a468bc7560..b1b846d8a094 100644
--- a/Documentation/userspace-api/seccomp_filter.rst
+++ b/Documentation/userspace-api/seccomp_filter.rst
@@ -122,6 +122,11 @@ In precedence order, they are:
 	Results in the lower 16-bits of the return value being passed
 	to userland as the errno without executing the system call.
 
+``SECCOMP_RET_USER_NOTIF``:
+    Results in a ``struct seccomp_notif`` message sent on the userspace
+    notification fd, if it is attached, or ``-ENOSYS`` if it is not. See below
+    on discussion of how to handle user notifications.
+
 ``SECCOMP_RET_TRACE``:
 	When returned, this value will cause the kernel to attempt to
 	notify a ``ptrace()``-based tracer prior to executing the system
@@ -183,6 +188,85 @@ The ``samples/seccomp/`` directory contains both an x86-specific example
 and a more generic example of a higher level macro interface for BPF
 program generation.
 
+Userspace Notification
+======================
+
+The ``SECCOMP_RET_USER_NOTIF`` return code lets seccomp filters pass a
+particular syscall to userspace to be handled. This may be useful for
+applications like container managers, which wish to intercept particular
+syscalls (``mount()``, ``finit_module()``, etc.) and change their behavior.
+
+To acquire a notification FD, use the ``SECCOMP_FILTER_FLAG_NEW_LISTENER``
+argument to the ``seccomp()`` syscall:
+
+.. code-block:: c
+
+    fd = seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_NEW_LISTENER, &prog);
+
+which (on success) will return a listener fd for the filter, which can then be
+passed around via ``SCM_RIGHTS`` or similar. Note that filter fds correspond to
+a particular filter, and not a particular task. So if this task then forks,
+notifications from both tasks will appear on the same filter fd. Reads and
+writes to/from a filter fd are also synchronized, so a filter fd can safely
+have many readers.
+
+The interface for a seccomp notification fd consists of two structures:
+
+.. code-block:: c
+
+    struct seccomp_notif_sizes {
+        __u16 seccomp_notif;
+        __u16 seccomp_notif_resp;
+        __u16 seccomp_data;
+    };
+
+    struct seccomp_notif {
+        __u64 id;
+        __u32 pid;
+        __u32 flags;
+        struct seccomp_data data;
+    };
+
+    struct seccomp_notif_resp {
+        __u64 id;
+        __s64 val;
+        __s32 error;
+        __u32 flags;
+    };
+
+The ``struct seccomp_notif_sizes`` structure can be used to determine the size
+of the various structures used in seccomp notifications. The size of ``struct
+seccomp_data`` may change in the future, so code should use:
+
+.. code-block:: c
+
+    struct seccomp_notif_sizes sizes;
+    seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes);
+
+to determine the size of the various structures to allocate. See
+samples/seccomp/user-trap.c for an example.
+
+Users can read via ``ioctl(SECCOMP_IOCTL_NOTIF_RECV)``  (or ``poll()``) on a
+seccomp notification fd to receive a ``struct seccomp_notif``, which contains
+five members: the input length of the structure, a unique-per-filter ``id``,
+the ``pid`` of the task which triggered this request (which may be 0 if the
+task is in a pid ns not visible from the listener's pid namespace), a ``flags``
+member which for now only has ``SECCOMP_NOTIF_FLAG_SIGNALED``, representing
+whether or not the notification is a result of a non-fatal signal, and the
+``data`` passed to seccomp. Userspace can then make a decision based on this
+information about what to do, and ``ioctl(SECCOMP_IOCTL_NOTIF_SEND)`` a
+response, indicating what should be returned to userspace. The ``id`` member of
+``struct seccomp_notif_resp`` should be the same ``id`` as in ``struct
+seccomp_notif``.
+
+It is worth noting that ``struct seccomp_data`` contains the values of register
+arguments to the syscall, but does not contain pointers to memory. The task's
+memory is accessible to suitably privileged traces via ``ptrace()`` or
+``/proc/pid/mem``. However, care should be taken to avoid the TOCTOU mentioned
+above in this document: all arguments being read from the tracee's memory
+should be read into the tracer's memory before any policy decisions are made.
+This allows for an atomic decision on syscall arguments.
+
 Sysctls
 =======