[PATCH net v2] vsock/vmci: fix UAF when peer resets connection during handshake

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

vmci_transport_recv_connecting_server() jumps to its destroy: label
and performs an unconditional sock_put(pending) to release the
explicit sock_hold() taken by vmci_transport_recv_listen() before
schedule_delayed_work().  The existing comment claimed this was safe
because the listen handler removes pending from the pending list on
the way out, which would prevent vsock_pending_work() from dropping
the same reference later.

That assumption breaks for a peer RST.  The default arm of the packet
switch sets:

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

and vmci_transport_recv_listen() only calls vsock_remove_pending()
when err < 0:

	if (err < 0)
		vsock_remove_pending(sk, pending);

For RST (err == 0) the socket stays on the pending list, so when
vsock_pending_work() fires it takes the is_pending=true path and
drops all three references itself: the pending-list reference via
vsock_remove_pending(), then the two trailing sock_put(sk) calls.
The unconditional sock_put() in destroy: had already dropped the
explicit sock_hold() reference, so the second trailing sock_put(sk)
in vsock_pending_work() is a write into the freed AF_VSOCK slab
object.  KASAN reports a slab-use-after-free write of 4 bytes from
refcount_warn_saturate() on the workqueue path:

  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
  Call Trace:
   refcount_warn_saturate
   vsock_pending_work
   process_one_work
   worker_thread

Triggering the bug requires only the ability to open a VSOCK
connection to the target and send a RST before the listener accepts.

Skip the sock_put() in destroy: when err == 0 so it only compensates
the cases where vmci_transport_recv_listen() actually calls
vsock_remove_pending().  RST is the only path that reaches destroy:
with err == 0; every other path produces a negative value, so their
behaviour is unchanged.

Verified on lts-6.12.79 with KASAN enabled (CONFIG_KASAN_INLINE=y,
kasan_multi_shot): same trigger binary, same VM, 100 iterations:
without this patch 52 KASAN slab-use-after-free reports fire; with
this patch applied, 0 reports.

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
---
v2:
  - Resubmit to netdev per Stefano Garzarella's request after v1 review.
  - Retested the PoC with the patch applied on lts-6.12.79 with KASAN
    enabled: 52/100 unpatched -> 0/100 patched (same trigger binary,
    same VM, 100 iterations); test summary captured in the commit
    message.
  - Changed Cc: stable@kernel.org -> stable@vger.kernel.org now that the
    bug is no longer embargoed.
  - Rebased onto net/main (no functional change to the diff).

v1 was sent to security@kernel.org on 2026-05-10 (not on lore archives;
no public link available).  v1 review summary, for reference:
  - Stefano Garzarella (vsock maintainer): "Overall LGTM, but I'd wait
    vmware guys on this that know this code better."  Asked for retest
    and resubmission via the net tree workflow.
  - Bryan Tan (VMCI maintainer): "Thanks for the fix, it looks good to
    me."  Also noted that no modern VMware product allows guest-to-guest
    VMCI communication, so the practical attack surface is host -> guest.

 net/vmw_vsock/vmci_transport.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/net/vmw_vsock/vmci_transport.c b/net/vmw_vsock/vmci_transport.c
index 4296ca1..88d7128 100644
--- a/net/vmw_vsock/vmci_transport.c
+++ b/net/vmw_vsock/vmci_transport.c
@@ -1269,14 +1269,16 @@ vmci_transport_recv_connecting_server(struct sock *listener,
 destroy:
 	pending->sk_err = skerr;
 	pending->sk_state = TCP_CLOSE;
-	/* As long as we drop our reference, all necessary cleanup will handle
-	 * when the cleanup function drops its reference and our destruct
-	 * implementation is called.  Note that since the listen handler will
-	 * remove pending from the pending list upon our failure, the cleanup
-	 * function won't drop the additional reference, which is why we do it
-	 * here.
+	/* Drop the reference taken by vmci_transport_recv_listen() before
+	 * schedule_delayed_work() only on real errors.  For a peer RST
+	 * (err == 0) the listener leaves pending on the pending list, and
+	 * vsock_pending_work() will drop that reference itself when it
+	 * later cleans the socket up.  Calling sock_put() here in that
+	 * case would be a double-put and free the socket while
+	 * vsock_pending_work() still holds it.
 	 */
-	sock_put(pending);
+	if (err < 0)
+		sock_put(pending);
 
 	return err;
 }

base-commit: be48e5fe51a5864566307998286a699d6b986934
-- 
2.54.0