From: "J. Bruce Fields" <bfields@redhat.com>
Date: Fri, 12 Apr 2019 16:37:30 -0400
Subject: [PATCH] nfsd: allow fh_want_write to be called twice
Git-commit: 0b8f62625dc309651d0efcb6a6247c933acd8b45
Patch-mainline: v5.2
References: git-fixes

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>
Acked-by: NeilBrown <neilb@suse.com>

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

--- a/fs/nfsd/vfs.h
+++ b/fs/nfsd/vfs.h
@@ -116,8 +116,11 @@ void		nfsd_put_raparams(struct file *fil
 
 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;