[PATCH net v4] vsock/virtio: clear vqs[] to avoid a UAF on a failed PM restore

[Jerome Mohm via B4 Relay <devnull+jrmmhm.kernel.eldare.de@kernel.org>]

From: Jerome Mohm <jrmmhm.kernel@eldare.de>

virtio_vsock_vqs_del() frees the virtqueues but leaves vsock->vqs[]
pointing at them. If virtio_vsock_restore() then fails, the driver
stays bound with a dangling vqs[], and the next freeze or unbind
passes a freed virtqueue to virtqueue_detach_unused_buf():

  BUG: KASAN: slab-use-after-free in virtqueue_detach_unused_buf
   virtio_vsock_vqs_del (net/vmw_vsock/virtio_transport.c:785)
   virtio_vsock_remove
  Freed by task ...:
   vp_del_vqs
   virtio_vsock_freeze

Clear vqs[] after del_vqs() and when virtio_find_vqs() fails. Return
early from virtio_vsock_vqs_del() when vqs[] is empty: the detach
loops would pass NULL to virtqueue_detach_unused_buf(), and a second
del_vqs() would free virtio-mmio's IRQ again, which only WARNs in
free_irq() ("Trying to free already-free IRQ").

Found with the help of Anthropic's Claude and reproduced on virtio-pci
and virtio-mmio by failing the restore with failslab, without other
kernel changes; the virtio-mmio WARN was found by the netdev Sashiko
review and reproduced with v1 of this patch. With this patch neither
report appears on virtio-pci or virtio-mmio guests, and the vsock
vmtest.sh single-VM tests pass. A find_vqs() failure after some
virtqueues were set up was not exercised.

Fixes: bd50c5dc182b ("vsock/virtio: add support for device suspend/resume")
Cc: stable@vger.kernel.org
Signed-off-by: Jerome Mohm <jrmmhm.kernel@eldare.de>
---
Changes in v4:
- Say why the early return is needed: the detach loops would pass NULL
  to virtqueue_detach_unused_buf() (answering Stefano).
- Say in the message and the comment that freeing virtio-mmio's IRQ
  again only WARNs (Stefano).
- Move the send queue purge before virtio_reset_device() (Stefano).
- Message: the use-after-free also reproduces on virtio-mmio.
- Rebase onto current net; rebuilt and retested on virtio-pci and
  virtio-mmio, and the vmtest.sh single-VM tests rerun on this version.
  The moved purge ran with an empty send queue only; the ns_* tests of
  vmtest.sh were not run (they need root on the host). The reproducers
  are available on request.
- Link to v3: https://patch.msgid.link/20261104-vsock-restore-uaf-send-v3-1-d60d06488b57@eldare.de
---
 net/vmw_vsock/virtio_transport.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index 4f9aa9c4c3aa..cf63bc99f3cc 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -714,8 +714,10 @@ static int virtio_vsock_vqs_init(struct virtio_vsock *vsock)
 	atomic_set(&vsock->queued_replies, 0);
 
 	ret = virtio_find_vqs(vdev, VSOCK_VQ_MAX, vsock->vqs, vqs_info, NULL);
-	if (ret < 0)
+	if (ret < 0) {
+		memset(vsock->vqs, 0, sizeof(vsock->vqs));
 		return ret;
+	}
 
 	virtio_vsock_update_guest_cid(vsock);
 
@@ -776,11 +778,19 @@ static void virtio_vsock_vqs_del(struct virtio_vsock *vsock)
 	vsock->event_run = false;
 	mutex_unlock(&vsock->event_lock);
 
+	virtio_vsock_skb_queue_purge(&vsock->send_pkt_queue);
+
 	/* Flush all device writes and interrupts, device will not use any
 	 * more buffers.
 	 */
 	virtio_reset_device(vdev);
 
+	/* After a failed restore no virtqueues are live, and a second
+	 * del_vqs() would free virtio-mmio's IRQ again (a WARN in free_irq()).
+	 */
+	if (!vsock->vqs[VSOCK_VQ_RX])
+		return;
+
 	mutex_lock(&vsock->rx_lock);
 	while ((skb = virtqueue_detach_unused_buf(vsock->vqs[VSOCK_VQ_RX])))
 		kfree_skb(skb);
@@ -791,10 +801,10 @@ static void virtio_vsock_vqs_del(struct virtio_vsock *vsock)
 		kfree_skb(skb);
 	mutex_unlock(&vsock->tx_lock);
 
-	virtio_vsock_skb_queue_purge(&vsock->send_pkt_queue);
-
 	/* Delete virtqueues and flush outstanding callbacks if any */
 	vdev->config->del_vqs(vdev);
+
+	memset(vsock->vqs, 0, sizeof(vsock->vqs));
 }
 
 static int virtio_vsock_probe(struct virtio_device *vdev)

---
base-commit: 1d5e2c57e9ffc134e14b3f837c8d7c53b508bd22
change-id: 20261001-vsock-restore-uaf-send-1fc7396fa1bd

Best regards,
--  
Jerome Mohm <jrmmhm.kernel@eldare.de>