[PATCH 0/2] plug capability races

[Mateusz Guzik]

fget_unlocked currently reads 'fde' which is a structure consisting of
serveral fields. In effect the read is inatomic and may result in
obtaining file pointer with stale or incorrect capabilities.

Example race is with dup2.

Side effect is that capability checks can be circumvented.

Proposed way to fix it is with the help of sequence counters.

Patchset assumes stuff from
'Getting rid of atomic_load_acq_int(&fdp->fd_nfiles)) from fget_unlocked'
( http://lists.freebsd.org/pipermail/freebsd-arch/2014-July/015550.html )
is applied. There is no technical dependency between patches (apart from
READ_ONCE), but this patch amortizes performance hit introduced with seqlock.

So this introduces a measurable hit with a microbenchmark (16 threads
reading from a pipe which fails with EAGAIN), but is still much faster than
current code with atomic_load_acq_int(&fdp->fd_nfiles).

x propernoacq-readpipe-run-sum
+ seq2-noacq-readpipe-run-sum
N           Min           Max        Median           Avg        Stddev
x  20      59479718      59527286      59496714      59499504     13752.968
+  20      54520752      54920054      54829539      54773480     136842.96
Difference at 95.0% confidence
    	-4.72602e+06 +/- 62244.4
	-7.94296% +/- 0.104613%
	(Student's t, pooled s = 97250)

There is still one theoretical race unfixed, but I don't believe it matters
much.

The race is:
fp gets reallocated before refcount check. this resuls in returning fp
regardless of new caps, but I don't see how this particular race could be
exploited. It could be fixed by re-reading entire fde and checking if it
changed.

-- 
2.0.2