xen/netfront: don't use gnttab_query_foreign_access() for mapped status

It isn't enough to check whether a grant is still being in use by
calling gnttab_query_foreign_access(), as a mapping could be realized
by the other side just after having called that function.

In case the call was done in preparation of revoking a grant it is
better to do so via gnttab_end_foreign_access_ref() and check the
success of that operation instead.

This is CVE-2022-23037 / part of XSA-396.

Reported-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
V2:
- use gnttab_try_end_foreign_access()
V3:
- don't use gnttab_try_end_foreign_access()

drivers/net/xen-netfront.c

@@ -424,14 +424,12 @@ static bool xennet_tx_buf_gc(struct netfront_queue *queue)
 			queue->tx_link[id] = TX_LINK_NONE;
 			skb = queue->tx_skbs[id];
 			queue->tx_skbs[id] = NULL;
-			if (unlikely(gnttab_query_foreign_access(
-				queue->grant_tx_ref[id]) != 0)) {
+			if (unlikely(!gnttab_end_foreign_access_ref(
+				queue->grant_tx_ref[id], GNTMAP_readonly))) {
 				dev_alert(dev,
 					  "Grant still in use by backend domain\n");
 				goto err;
 			}
-			gnttab_end_foreign_access_ref(
-				queue->grant_tx_ref[id], GNTMAP_readonly);
 			gnttab_release_grant_reference(
 				&queue->gref_tx_head, queue->grant_tx_ref[id]);
 			queue->grant_tx_ref[id] = GRANT_INVALID_REF;