[PATCH bpf] bpf, sockmap: Fix af_unix null-ptr-deref in proto update

[Michal Luczaj]

BPF_MAP_UPDATE_ELEM races unix_stream_connect(): when
sock_map_sk_state_allowed() passes (sk_state == TCP_ESTABLISHED),
unix_peer(sk) in unix_stream_bpf_update_proto() may still return NULL.

	T0 bpf				T1 connect
	------				----------

				WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED)
sock_map_sk_state_allowed(sk)
...
sk_pair = unix_peer(sk)
sock_hold(sk_pair)
				sock_hold(newsk)
				smp_mb__after_atomic()
				unix_peer(sk) = newsk

BUG: kernel NULL pointer dereference, address: 0000000000000080
RIP: 0010:unix_stream_bpf_update_proto+0xa0/0x1b0
Call Trace:
 sock_map_link+0x564/0x8b0
 sock_map_update_common+0x6e/0x340
 sock_map_update_elem_sys+0x17d/0x240
 __sys_bpf+0x26db/0x3250
 __x64_sys_bpf+0x21/0x30
 do_syscall_64+0x6b/0x3a0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Follow-up to discussion at
https://lore.kernel.org/netdev/20240610174906.32921-1-kuniyu@xxxxxxxxxx/.

Fixes: 8866730aed51 ("bpf, sockmap: af_unix stream sockets need to hold ref for pair sock")
Suggested-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
Signed-off-by: Michal Luczaj <mhal@xxxxxxx>
---
Re-triggered while working on an unrelated selftest:
https://lore.kernel.org/bpf/20260123-selftest-signal-on-connect-v1-0-b0256e7025b6@xxxxxxx/
---
 net/unix/unix_bpf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/unix/unix_bpf.c b/net/unix/unix_bpf.c
index e0d30d6d22ac..57f3124c9d8d 100644
--- a/net/unix/unix_bpf.c
+++ b/net/unix/unix_bpf.c
@@ -185,6 +185,9 @@ int unix_stream_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool r
 	 */
 	if (!psock->sk_pair) {
 		sk_pair = unix_peer(sk);
+		if (unlikely(!sk_pair))
+			return -EINVAL;
+
 		sock_hold(sk_pair);
 		psock->sk_pair = sk_pair;
 	}

---
base-commit: 63804fed149a6750ffd28610c5c1c98cce6bd377
change-id: 20260129-unix-proto-update-null-ptr-deref-6a2733bcbbf8

Best regards,
-- 
Michal Luczaj <mhal@xxxxxxx>