Commit 0b8f6262 authored Apr 12, 2019 by J. Bruce Fields

nfsd: allow fh_want_write to be called twice



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>

parent e45d1a18

fs/nfsd/vfs.h

+4 −1

Original line number	Diff line number	Diff line
		@@ -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;

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