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

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

[J. Bruce Fields]

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

[PATCH 1/2] nfsd: allow fh_want_write to be called twice

[J. Bruce Fields]

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

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>
---
 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;
-- 
2.21.0

[PATCH 2/2] nfsd: fh_drop_write in nfsd_unlink

[J. Bruce Fields]

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

fh_want_write() can now be called twice, but I'm also fixing up the
callers not to do that.

Other cases include setattr and create.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
---
 fs/nfsd/vfs.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 7dc98e14655d..fc24ee47eab5 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1786,12 +1786,12 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
 	rdentry = lookup_one_len(fname, dentry, flen);
 	host_err = PTR_ERR(rdentry);
 	if (IS_ERR(rdentry))
-		goto out_nfserr;
+		goto out_drop_write;
 
 	if (d_really_is_negative(rdentry)) {
 		dput(rdentry);
-		err = nfserr_noent;
-		goto out;
+		host_err = -ENOENT;
+		goto out_drop_write;
 	}
 
 	if (!type)
@@ -1805,6 +1805,8 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
 		host_err = commit_metadata(fhp);
 	dput(rdentry);
 
+out_drop_write:
+	fh_drop_write(fhp);
 out_nfserr:
 	err = nfserrno(host_err);
 out:
-- 
2.21.0