vfs: fix subtle use-after-free of pipe_inode_info
The pipe code was trying (and failing) to be very careful about freeing the pipe info only after the last access, with a pattern like: spin_lock(&inode->i_lock); if (!--pipe->files) { inode->i_pipe = NULL; kill = 1; } spin_unlock(&inode->i_lock); __pipe_unlock(pipe); if (kill) free_pipe_info(pipe); where the final freeing is done last. HOWEVER. The above is actually broken, because while the freeing is done at the end, if we have two racing processes releasing the pipe inode info, the one that *doesn't* free it will decrement the ->files count, and unlock the inode i_lock, but then still use the "pipe_inode_info" afterwards when it does the "__pipe_unlock(pipe)". This is *very* hard to trigger in practice, since the race window is very small, and adding debug options seems to just hide it by slowing things down. Simon originally reported this way back in July as an Oops in kmem_cache_allocate due to a single bit corruption (due to the final "spin_unlock(pipe->mutex.wait_lock)" incrementing a field in a different allocation that had re-used the free'd pipe-info), it's taken this long to figure out. Since the 'pipe->files' accesses aren't even protected by the pipe lock (we very much use the inode lock for that), the simple solution is to just drop the pipe lock early. And since there were two users of this pattern, create a helper function for it. Introduced commit ba5bb147 ("pipe: take allocation and freeing of pipe_inode_info out of ->i_mutex"). Reported-by: NSimon Kirby <sim@hostway.ca> Reported-by: NIan Applegate <ia@cloudflare.com> Acked-by: NAl Viro <viro@zeniv.linux.org.uk> Cc: stable@kernel.org # v3.10+ Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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