[PATCH] nfsd: fix oops on mixed NFSv4/NFSv3 client access

[Bruce Fields <bfields@fieldses.org>]

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

If an NFSv2/v3 client breaks an NFSv4 client's delegation, it will hit a
NULL dereference in nfsd_breaker_owns_lease().

Easily reproduceable with for example

	mount -overs=4.2 server:/export /mnt/
	sleep 1h </mnt/file &
	mount -overs=3 server:/export /mnt2/
	touch /mnt2/file

Reported-by: Robert Dinse <nanook@eskimo.com>
Fixes: 28df3d1539de50 ("nfsd: clients don't need to break their own delegations")
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
---
 fs/nfsd/nfs4state.c | 2 ++
 1 file changed, 2 insertions(+)

Oops, I've had this sitting around a couple weeks but I must have
forgotten to send it in.  This is needed for 5.9.

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 4b70657385f2..0e5302f6df52 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -4598,6 +4598,8 @@ static bool nfsd_breaker_owns_lease(struct file_lock *fl)
 	if (!i_am_nfsd())
 		return NULL;
 	rqst = kthread_data(current);
+	if (!rqst->rq_lease_breaker)
+		return NULL;
 	clp = *(rqst->rq_lease_breaker);
 	return dl->dl_stid.sc_client == clp;
 }
-- 
2.26.2