From 5e8a754ac2009a88a7b99ab61eab6296eed360f3 Mon Sep 17 00:00:00 2001 From: David Lee Date: Mon, 13 Jul 2026 10:47:50 +0000 Subject: [PATCH] net/x25: fix use-after-free in x25_kill_by_neigh() commit 5499e0602d2faafd42c580d25f615903c3fbe11b upstream. x25_kill_by_neigh() walks the global X.25 socket list looking for sockets attached to a terminating neighbour. x25_list_lock protects list membership while the lookup is in progress, but it does not pin a socket's lifetime after the lock is dropped. The function currently drops x25_list_lock before calling lock_sock(s). A concurrent close can run x25_release(), remove the same socket from x25_list, and drop the last socket reference in that window. The neighbour teardown path can then lock or inspect a freed struct sock/struct x25_sock. Take sock_hold(s) while x25_list_lock still proves that the list entry is live, then drop the temporary reference after the socket has been locked, rechecked, and released. Recheck x25_sk(s)->neighbour after lock_sock(), because another path may have disconnected the socket before this path acquired the socket lock. Restart the list walk after each disconnect because the list lock was dropped and the previous iterator state may no longer be valid. A QEMU/KASAN run against origin/master reproduced a slab-use-after-free in x25_kill_by_neigh(). Fixes: 7781607938c8 ("net/x25: Fix null-ptr-deref caused by x25_disconnect") Cc: stable@vger.kernel.org Signed-off-by: David Lee Assisted-by: Codex:gpt-5.5 Acked-by: Martin Schiller Link: https://patch.msgid.link/20260713104752.241175-1-david.lee@trailofbits.com Signed-off-by: Paolo Abeni Signed-off-by: Greg Kroah-Hartman --- net/x25/af_x25.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/net/x25/af_x25.c b/net/x25/af_x25.c index 1e5152fdee58..cdc5b4bf5b43 100644 --- a/net/x25/af_x25.c +++ b/net/x25/af_x25.c @@ -1769,15 +1769,19 @@ void x25_kill_by_neigh(struct x25_neigh *nb) { struct sock *s; +again: write_lock_bh(&x25_list_lock); sk_for_each(s, &x25_list) { if (x25_sk(s)->neighbour == nb) { + sock_hold(s); write_unlock_bh(&x25_list_lock); lock_sock(s); - x25_disconnect(s, ENETUNREACH, 0, 0); + if (x25_sk(s)->neighbour == nb) + x25_disconnect(s, ENETUNREACH, 0, 0); release_sock(s); - write_lock_bh(&x25_list_lock); + sock_put(s); + goto again; } } write_unlock_bh(&x25_list_lock);