From: Jan Beulich <redacted>

commit 2991397d23ec597405b116d96de3813420bdcbc3 upstream.

Commit 3194a1746e8a ("xen-netback: don't "handle" error by BUG()")
dropped respective a BUG_ON() without noticing that with this the
variable's value wouldn't be consumed anymore. With gnttab_set_map_op()
setting all status fields to a non-zero value, in case of an error no
slot should have a status of GNTST_okay (zero).

This is part of XSA-367.

Cc: <redacted>
Reported-by: kernel test robot <redacted>
Signed-off-by: Jan Beulich <redacted>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/d933f495-619a-0086-5fb4-1ec3cf81a8fc@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/net/xen-netback/netback.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c

@@ -1326,11 +1326,21 @@ int xenvif_tx_action(struct xenvif_queue
 		return 0;
 
 	gnttab_batch_copy(queue->tx_copy_ops, nr_cops);
-	if (nr_mops != 0)
+	if (nr_mops != 0) {
 		ret = gnttab_map_refs(queue->tx_map_ops,
 				      NULL,
 				      queue->pages_to_map,
 				      nr_mops);
+		if (ret) {
+			unsigned int i;
+
+			netdev_err(queue->vif->dev, "Map fail: nr %u ret %d\n",
+				   nr_mops, ret);
+			for (i = 0; i < nr_mops; ++i)
+				WARN_ON_ONCE(queue->tx_map_ops[i].status ==
+				             GNTST_okay);
+		}
+	}
 
 	work_done = xenvif_tx_submit(queue);