[PATCH 0/2] nfsd shoudn't call mnt_want_write twice

From: "J. Bruce Fields" <bfields@redhat.com>
To: linux-nfs@vger.kernel.org
Cc: "J. Bruce Fields" <bfields@redhat.com>
Subject: [PATCH 0/2] nfsd shoudn't call mnt_want_write twice
Date: Mon, 13 May 2019 11:27:01 -0400	[thread overview]
Message-ID: <1557761223-14483-1-git-send-email-bfields@redhat.com> (raw)

From: "J. Bruce Fields" <bfields@redhat.com>

A fuzzer recently triggered lockdep warnings about potential sb_writers
deadlocks caused by fh_want_write, showing that we haven't been careful
to pair each fh_want_write() with an fh_drop_write().

This isn't normally a problem since fh_put() will call fh_drop_write()
for us.  And that's OK for NFSv3.  But an NFSv4 protocol fuzzer can do
weird things like call unlink twice in a compound.

So we can either make it safe to call fh_want_write() twice, or we can
fix all the callers to call fh_drop_write().

For now I think we have to do the former just to get the bug fixed.

Long term I don't know whether it's best to stay with that or to fix up
the callers.  I fixed nfsd_unlink just because it's easy, but maybe
that's pointless, it's others (like setattr) that are the complicated
ones.

J. Bruce Fields (2):
  nfsd: allow fh_want_write to be called twice
  nfsd: fh_drop_write in nfsd_unlink

 fs/nfsd/vfs.c | 8 +++++---
 fs/nfsd/vfs.h | 5 ++++-
 2 files changed, 9 insertions(+), 4 deletions(-)

-- 
2.21.0