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Move a racy assertion in filt_pipewrite().

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EVFILT_WRITE knotes for pipes live on the knlist for the other end of the
pipe.  Since they do not hold a reference on the corresponding file
structure, they may be removed from the knlist by pipeclose() while still
remaining active.  In this case, there is no knlist lock acquired before
filt_pipewrite() is called, so the assertion fails.

Fix the problem by first checking whether that end of the pipe has been
closed.  These checks are memory safe since the knote holds a reference
on one end of the pipe, and the pipe structure is not freed until both
ends are closed.  The checks are not racy since PIPE_EOF is never cleared
after being set, and pipe_present is never set back to PIPE_ACTIVE after
pipeclose() has been called.

PR:		235640
Reported and tested by:	pho
Reviewed by:	kib
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D19224
This commit is contained in:
Mark Johnston 2019-02-19 15:46:43 +00:00
parent c9172fb4f1
commit 18a7de663b
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=344278
1 changed files with 6 additions and 2 deletions
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8
sys/kern/sys_pipe.c
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@ -1741,15 +1741,19 @@ static int
filt_pipewrite(struct knote *kn, long hint)
{
struct pipe *wpipe;
/*
* If this end of the pipe is closed, the knote was removed from the
* knlist and the list lock (i.e., the pipe lock) is therefore not held.
*/
wpipe = kn->kn_hook;
PIPE_LOCK_ASSERT(wpipe, MA_OWNED);
if (wpipe->pipe_present != PIPE_ACTIVE ||
(wpipe->pipe_state & PIPE_EOF)) {
kn->kn_data = 0;
kn->kn_flags |= EV_EOF;
return (1);
}
PIPE_LOCK_ASSERT(wpipe, MA_OWNED);
kn->kn_data = (wpipe->pipe_buffer.size > 0) ?
(wpipe->pipe_buffer.size - wpipe->pipe_buffer.cnt) : PIPE_BUF;
if (wpipe->pipe_state & PIPE_DIRECTW)