[PATCH 7.2.y 3/6] NFSD: Prevent client use-after-free during export state revocation

[Sasha Levin <sashal@kernel.org>]

From: Chuck Lever <cel@kernel.org>

[ Upstream commit 2108de53568a64936a0da3e04d85c35df98d3fb6 ]

nfsd4_revoke_export_states() has the same use-after-free as
nfsd4_revoke_states(): it drops nn->client_lock across
revoke_one_stid() and the following read of clp->cl_minorversion, but
the stateid reference it holds does not pin the client.  A teardown
racing the dropped lock can free the client while revoke_one_stid()
still dereferences it.

exportfs -u drives this path through NFSD_CMD_UNLOCK_EXPORT, so an
administrator removing an export can race a client expiry.

Skip a client that is already expiring and otherwise pin it with
cl_rpc_users under client_lock before dropping the lock, matching
nfsd4_revoke_states().

Fixes: 2eac189bb059 ("NFSD: Add NFSD_CMD_UNLOCK_EXPORT netlink command")
Reviewed-by: NeilBrown <neil@brown.name>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260709-cel-v4-4-1d519d9be0cb@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Stable-dep-of: 2330b788d732 ("NFSD: Prevent client use-after-free during close_lru reaping")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/nfsd/nfs4state.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 8b0feceee5ac6..fd97042abbb2e 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -1973,10 +1973,14 @@ void nfsd4_revoke_export_states(struct nfsd_net *nn, const struct path *path)
 		struct nfs4_client *clp;
 	retry:
 		list_for_each_entry(clp, head, cl_idhash) {
-			struct nfs4_stid *stid = find_one_export_stid(
-							clp, path,
-							sc_types);
+			struct nfs4_stid *stid;
+
+			/* Skip or pin clp as in nfsd4_revoke_states(). */
+			if (is_client_expired(clp))
+				continue;
+			stid = find_one_export_stid(clp, path, sc_types);
 			if (stid) {
+				atomic_inc(&clp->cl_rpc_users);
 				spin_unlock(&nn->client_lock);
 				revoke_one_stid(nn, clp, stid);
 				nfs4_put_stid(stid);
@@ -1984,6 +1988,9 @@ void nfsd4_revoke_export_states(struct nfsd_net *nn, const struct path *path)
 				if (clp->cl_minorversion == 0)
 					nn->nfs40_last_revoke =
 						ktime_get_boottime_seconds();
+				if (atomic_dec_and_test(&clp->cl_rpc_users) &&
+				    is_client_expired(clp))
+					wake_up_all(&expiry_wq);
 				goto retry;
 			}
 		}
-- 
2.53.0