Improve locking of pipe mutexes in the context of MAC:
(1) Where previously the pipe mutex was selectively grabbed during
pipe_ioctl(), now always grab it and then release if if not needed. This protects the call to mac_check_pipe_ioctl() to make sure the label remains consistent. (Note: it looks like sigio locking may be incorrect for fgetown() since we call it not-by-reference and sigio locking assumes call by reference).
(2) In pipe_stat(), lock the pipe if MAC is compiled in so that
the call to mac_check_pipe_stat() gets a locked pipe to protect label consistency. We still release the lock before returning actual stat() data, risking inconsistency, but apparently our pipe locking model accepts that risk.
(3) In various pipe MAC authorization checks, assert that the pipe
lock is held.
(4) Grab the lock when performing a pipe relabel operation, and
assert it a little deeper in the stack.
Obtained from: TrustedBSD Project
Sponsored by: DARPA, Network Associates Laboratories