[PATCH 5.1 086/155] nfsd: allow fh_want_write to be called twice

Greg Kroah-Hartman
2019-06-13T16:02:06Z

[ 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>
---
fs/nfsd/vfs.h | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/vfs.h b/fs/nfsd/vfs.h
index a7e107309f76..db351247892d 100644
--- a/fs/nfsd/vfs.h
+++ b/fs/nfsd/vfs.h
@@ -120,8 +120,11 @@ void nfsd_put_raparams(struct file *file, struct raparms *ra);

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)
fh->fh_want_write = true;
return ret;
--
2.20.1