[PATCH net] net/x25: fix use-after-free in x25_kill_by_neigh()

[David Lee <david.lee@trailofbits.com>]

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 <david.lee@trailofbits.com>
Assisted-by: Codex:gpt-5.5
---
Trail of Bits has a reproducer that triggers kernel panic demonstrating the bug, and can share it if needed.

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 c31d2af5dd22..8aae9273b7c1 100644
--- a/net/x25/af_x25.c
+++ b/net/x25/af_x25.c
@@ -1768,15 +1768,19 @@ 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);