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nfsd: allow fh_want_write to be called twice

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[ Upstream commit 0b8f62625dc309651d0efcb6a6247c933acd8b45 ]

A fuzzer recently triggered lockdep warnings about potential sb_writers
deadlocks caused by fh_want_write().

Looks like we aren't careful to pair each fh_want_write() with an
fh_drop_write().

It's not normally a problem since fh_put() will call fh_drop_write() for
us.  And was OK for NFSv3 where we'd do one operation that might call
fh_want_write(), and then put the filehandle.

But an NFSv4 protocol fuzzer can do weird things like call unlink twice
in a compound, and then we get into trouble.

I'm a little worried about this approach of just leaving everything to
fh_put().  But I think there are probably a lot of
fh_want_write()/fh_drop_write() imbalances so for now I think we need it
to be more forgiving.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
J. Bruce Fields 2019-04-12 16:37:30 -04:00 committed by Greg Kroah-Hartman
parent 1e0a2528fb
commit 76f53b8464
1 changed files with 4 additions and 1 deletions
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5
fs/nfsd/vfs.h
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@ -117,8 +117,11 @@ void nfsd_put_raparams(struct file *file, struct raparms *ra);
static inline int fh_want_write(struct svc_fh *fh) static inline int fh_want_write(struct svc_fh *fh)
{ {
int ret = mnt_want_write(fh->fh_export->ex_path.mnt); int ret;
if (fh->fh_want_write)
return 0;
ret = mnt_want_write(fh->fh_export->ex_path.mnt);
if (!ret) if (!ret)
fh->fh_want_write = true; fh->fh_want_write = true;
return ret; return ret;