seccomp: Add wait_killable semantic to seccomp user notifier

This introduces a per-filter flag (SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV)
that makes it so that when notifications are received by the supervisor the
notifying process will transition to wait killable semantics. Although wait
killable isn't a set of semantics formally exposed to userspace, the
concept is searchable. If the notifying process is signaled prior to the
notification being received by the userspace agent, it will be handled as
normal.

One quirk about how this is handled is that the notifying process
only switches to TASK_KILLABLE if it receives a wakeup from either
an addfd or a signal. This is to avoid an unnecessary wakeup of
the notifying task.

The reasons behind switching into wait_killable only after userspace
receives the notification are:
* Avoiding unncessary work - Often, workloads will perform work that they
  may abort (request racing comes to mind). This allows for syscalls to be
  aborted safely prior to the notification being received by the
  supervisor. In this, the supervisor doesn't end up doing work that the
  workload does not want to complete anyways.
* Avoiding side effects - We don't want the syscall to be interruptible
  once the supervisor starts doing work because it may not be trivial
  to reverse the operation. For example, unmounting a file system may
  take a long time, and it's hard to rollback, or treat that as
  reentrant.
* Avoid breaking runtimes - Various runtimes do not GC when they are
  during a syscall (or while running native code that subsequently
  calls a syscall). If many notifications are blocked, and not picked
  up by the supervisor, this can get the application into a bad state.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220503080958.20220-2-sargun@sargun.me

1 files changed, 10 insertions, 0 deletions

diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
index 539e9d4a4860..d1e2b9193f09 100644
--- a/Documentation/userspace-api/seccomp_filter.rst
+++ b/Documentation/userspace-api/seccomp_filter.rst
@@ -271,6 +271,16 @@ notifying process it will be replaced. The supervisor can also add an FD, and
 respond atomically by using the ``SECCOMP_ADDFD_FLAG_SEND`` flag and the return
 value will be the injected file descriptor number.
 
+The notifying process can be preempted, resulting in the notification being
+aborted. This can be problematic when trying to take actions on behalf of the
+notifying process that are long-running and typically retryable (mounting a
+filesytem). Alternatively, at filter installation time, the
+``SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV`` flag can be set. This flag makes it
+such that when a user notification is received by the supervisor, the notifying
+process will ignore non-fatal signals until the response is sent. Signals that
+are sent prior to the notification being received by userspace are handled
+normally.
+
 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