[PATCH net v3] vsock/vmci: fix UAF when peer resets connection during handshake

[Minh Nguyen <minhnguyen.080505@gmail.com>]

vmci_transport_recv_connecting_server() returned err = 0 for a peer
RST in its default switch arm:

	err = pkt->type == VMCI_TRANSPORT_PACKET_TYPE_RST ? 0 : -EINVAL;

That made vmci_transport_recv_listen() skip vsock_remove_pending(),
leaving the pending socket on the listener's pending_links with
sk_state = TCP_CLOSE while destroy: still dropped the explicit
reference taken before schedule_delayed_work().

One second later vsock_pending_work() observed is_pending=true and
performed full cleanup: vsock_remove_pending() then the two trailing
sock_put(sk) calls -- the first reached refcount 0 and __sk_freed
the socket, and the second wrote into the freed object:

  BUG: KASAN: slab-use-after-free in refcount_warn_saturate
  Write of size 4 at addr ffff88800b1cac80 by task kworker
  Workqueue: events vsock_pending_work

Treat peer RST like any other unexpected packet type (err = -EINVAL).
All destroy: arms now return err < 0, so vmci_transport_recv_listen()
removes pending from pending_links synchronously and
vsock_pending_work() takes the is_pending=false / !rejected branch,
dropping only its own work reference.  This also closes the
multi-packet race Sashiko reported on v2: pending is removed from
the list before any subsequent packet can find it.

The pre-existing sk_acceptq_removed() gap on the err < 0 path of
vmci_transport_recv_listen() that Sashiko also noted is not
introduced or changed by this patch.

Tested on lts-6.12.79 with KASAN: 52/100 unpatched -> 0/100 patched.

Fixes: d021c344051a ("VSOCK: Introduce VM Sockets")
Cc: stable@vger.kernel.org
Signed-off-by: Minh Nguyen <minhnguyen.080505@gmail.com>
Assisted-by: Claude:claude-opus-4-7
---
v3:
  - Different approach to Sashiko/Paolo's "trading UAF for leak"
    concern: normalize RST to err = -EINVAL so all destroy: arms
    take the same err < 0 cleanup path -- no special case, no
    multi-packet race.
  - Sashiko's secondary observation ("while not introduced by this
    patch, does this error path leak sk_ack_backlog slots on failed
    handshakes?") is correct: the sk_acceptq_removed() gap on the
    err < 0 branch of vmci_transport_recv_listen() is pre-existing
    and is not introduced or changed by this patch.  v3 stays
    focused on the UAF; a separate fix for that gap is needed and
    would be welcome from anyone closer to that area.

v2: https://lore.kernel.org/netdev/20260512025851.189140-1-minhnguyen.080505@gmail.com/

v1 was sent to security@kernel.org on 2026-05-10 (not on lore).

 net/vmw_vsock/vmci_transport.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/net/vmw_vsock/vmci_transport.c b/net/vmw_vsock/vmci_transport.c
index 4296ca1..ba3a66e 100644
--- a/net/vmw_vsock/vmci_transport.c
+++ b/net/vmw_vsock/vmci_transport.c
@@ -1161,10 +1161,17 @@ vmci_transport_recv_connecting_server(struct sock *listener,
 		}
 		break;
 	default:
-		/* Close and cleanup the connection. */
+		/* Close and cleanup the connection.  Peer RST is treated like
+		 * any other unexpected packet type in this state so that the
+		 * pending socket follows the same cleanup path as other
+		 * handshake failures, instead of being left on the pending
+		 * list for vsock_pending_work() to find later (which races
+		 * with subsequent packets and was the source of a UAF when
+		 * the cleanup work observed an inconsistent ref count).
+		 */
 		vmci_transport_send_reset(pending, pkt);
 		skerr = EPROTO;
-		err = pkt->type == VMCI_TRANSPORT_PACKET_TYPE_RST ? 0 : -EINVAL;
+		err = -EINVAL;
 		goto destroy;
 	}
 

base-commit: be48e5fe51a5864566307998286a699d6b986934
-- 
2.54.0