[PATCH 7.2.y 4/6] NFSD: Prevent client use-after-free during NFSv4.0 revoked-state cleanup

[Sasha Levin <sashal@kernel.org>]

From: Chuck Lever <cel@kernel.org>

[ Upstream commit 7b4f8a1586c42d3afc3c0ac779af2db7ab1a5c55 ]

nfs40_clean_admin_revoked() takes a stateid reference under
clp->cl_lock, drops nn->client_lock, and calls
nfsd4_drop_revoked_stid(), which dereferences the stateid's client
through s->sc_client->cl_lock.  The stateid reference does not pin the
client, so a teardown racing the dropped lock can free the client
while nfsd4_drop_revoked_stid() is still using it.

This cleanup runs from the laundromat, so a periodic sweep can race
force_expire_client() driven by a write to the clients/<id>/ctl file.

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: d688d8585e6b ("nfsd: allow admin-revoked NFSv4.0 state to be freed.")
Cc: stable@vger.kernel.org
Reviewed-by: NeilBrown <neil@brown.name>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260709-cel-v4-5-1d519d9be0cb@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Stable-dep-of: 9026932ac8be ("NFSD: Prevent client use-after-free during blocked-lock reaping")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/nfsd/nfs4state.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index fd97042abbb2e..e26ea5b6295c2 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -7057,16 +7057,22 @@ static void nfs40_clean_admin_revoked(struct nfsd_net *nn,
 
 		if (atomic_read(&clp->cl_admin_revoked) == 0)
 			continue;
+		if (is_client_expired(clp))
+			continue;
 
 		spin_lock(&clp->cl_lock);
 		idr_for_each_entry_ul(&clp->cl_stateids, stid, tmp, id)
 			if (stid->sc_status & SC_STATUS_ADMIN_REVOKED) {
 				refcount_inc(&stid->sc_count);
+				atomic_inc(&clp->cl_rpc_users);
 				spin_unlock(&nn->client_lock);
 				/* this function drops ->cl_lock */
 				nfsd4_drop_revoked_stid(stid);
 				nfs4_put_stid(stid);
 				spin_lock(&nn->client_lock);
+				if (atomic_dec_and_test(&clp->cl_rpc_users) &&
+				    is_client_expired(clp))
+					wake_up_all(&expiry_wq);
 				goto retry;
 			}
 		spin_unlock(&clp->cl_lock);
-- 
2.53.0