From 9026932ac8be4d0ae01db47f23619a98cc57b671 Mon Sep 17 00:00:00 2001 From: Chuck Lever Date: Thu, 9 Jul 2026 13:40:30 -0400 Subject: [PATCH] NFSD: Prevent client use-after-free during blocked-lock reaping A bare lock owner -- its only remaining reference a blocked lock on nn->blocked_locks_lru -- holds a raw pointer to its nfs4_client but no reference keeping the client alive. When the per-net laundromat reaps such a lock, freeing the nbl drops the owner reference held through flc_owner, and the final nfs4_put_stateowner() takes the client's cl_lock. Because the laundromat detaches the nbl first, __destroy_client() no longer finds it, so a concurrent force_expire_client() can free the client before nfs4_put_stateowner() runs, dereferencing cl_lock in freed memory. Pin the client with cl_rpc_users before dropping nn->blocked_locks_lock, and skip clients already expiring, whose blocked locks __destroy_client() frees while holding an owner reference. Take nn->client_lock outside nn->blocked_locks_lock. Every other site holds nn->blocked_locks_lock as a leaf, acquiring no further lock, so placing nn->client_lock outside it cannot form a lock-order cycle. Fixes: 7919d0a27f1e ("nfsd: add a LRU list for blocked locks") Cc: stable@vger.kernel.org Reviewed-by: Jeff Layton Link: https://patch.msgid.link/20260709-cel-v4-7-1d519d9be0cb@kernel.org Signed-off-by: Chuck Lever --- fs/nfsd/nfs4state.c | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c index 142ba7d80539..4acd02f1642c 100644 --- a/fs/nfsd/nfs4state.c +++ b/fs/nfsd/nfs4state.c @@ -357,6 +357,16 @@ free_blocked_lock(struct nfsd4_blocked_lock *nbl) kref_put(&nbl->nbl_kref, free_nbl); } +/* A blocked lock's flc_owner is its nfs4_lockowner. */ +static struct nfs4_client * +nbl_client(struct nfsd4_blocked_lock *nbl) +{ + struct nfs4_lockowner *lo; + + lo = (struct nfs4_lockowner *)nbl->nbl_lock.c.flc_owner; + return lo->lo_owner.so_client; +} + static void remove_blocked_locks(struct nfs4_lockowner *lo) { @@ -7591,22 +7601,29 @@ nfs4_laundromat(struct nfsd_net *nn) * indefinitely once the lock does become free. */ BUG_ON(!list_empty(&reaplist)); + spin_lock(&nn->client_lock); spin_lock(&nn->blocked_locks_lock); - while (!list_empty(&nn->blocked_locks_lru)) { - nbl = list_first_entry(&nn->blocked_locks_lru, - struct nfsd4_blocked_lock, nbl_lru); + list_for_each_safe(pos, next, &nn->blocked_locks_lru) { + nbl = list_entry(pos, struct nfsd4_blocked_lock, nbl_lru); if (!state_expired(<, nbl->nbl_time)) break; + clp = nbl_client(nbl); + if (is_client_expired(clp)) + continue; + atomic_inc(&clp->cl_rpc_users); list_move(&nbl->nbl_lru, &reaplist); list_del_init(&nbl->nbl_list); } spin_unlock(&nn->blocked_locks_lock); + spin_unlock(&nn->client_lock); while (!list_empty(&reaplist)) { nbl = list_first_entry(&reaplist, struct nfsd4_blocked_lock, nbl_lru); + clp = nbl_client(nbl); list_del_init(&nbl->nbl_lru); free_blocked_lock(nbl); + put_client_no_renew(clp); } #ifdef CONFIG_NFSD_V4_2_INTER_SSC /* service the server-to-server copy delayed unmount list */