seccomp: release filter after task is fully dead

The seccomp filter used to be released in free_task() which is called
asynchronously via call_rcu() and assorted mechanisms. Since we need
to inform tasks waiting on the seccomp notifier when a filter goes empty
we will notify them as soon as a task has been marked fully dead in
release_task(). To not split seccomp cleanup into two parts, move
filter release out of free_task() and into release_task() after we've
unhashed struct task from struct pid, exited signals, and unlinked it
from the threadgroups' thread list. We'll put the empty filter
notification infrastructure into it in a follow up patch.

This also renames put_seccomp_filter() to seccomp_filter_release() which
is a more descriptive name of what we're doing here especially once
we've added the empty filter notification mechanism in there.

We're also NULL-ing the task's filter tree entrypoint which seems
cleaner than leaving a dangling pointer in there. Note that this shouldn't
need any memory barriers since we're calling this when the task is in
release_task() which means it's EXIT_DEAD. So it can't modify its seccomp
filters anymore. You can also see this from the point where we're calling
seccomp_filter_release(). It's after __exit_signal() and at this point,
tsk->sighand will already have been NULLed which is required for
thread-sync and filter installation alike.

Cc: Tycho Andersen <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Matt Denton <[email protected]>
Cc: Sargun Dhillon <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Chris Palmer <[email protected]>
Cc: Aleksa Sarai <[email protected]>
Cc: Robert Sesek <[email protected]>
Cc: Jeffrey Vander Stoep <[email protected]>
Cc: Linux Containers <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

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