[PATCH 1/1] xen-netfront: do not assume sk_buff_head list is empty in error handling

From: Dongli Zhang
Date: Sun Sep 15 2019 - 23:46:43 EST

When skb_shinfo(skb) is not able to cache extra fragment (that is,
skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS), xennet_fill_frags() assumes
the sk_buff_head list is already empty. As a result, cons is increased only
by 1 and returns to error handling path in xennet_poll().

However, if the sk_buff_head list is not empty, queue->rx.rsp_cons may be
set incorrectly. That is, queue->rx.rsp_cons would point to the rx ring
buffer entries whose queue->rx_skbs[i] and queue->grant_rx_ref[i] are
already cleared to NULL. This leads to NULL pointer access in the next
iteration to process rx ring buffer entries.

Below is how xennet_poll() does error handling. All remaining entries in
tmpq are accounted to queue->rx.rsp_cons without assuming how many
outstanding skbs are remained in the list.

985 static int xennet_poll(struct napi_struct *napi, int budget)
... ...
1032 if (unlikely(xennet_set_skb_gso(skb, gso))) {
1033 __skb_queue_head(&tmpq, skb);
1034 queue->rx.rsp_cons += skb_queue_len(&tmpq);
1035 goto err;
1036 }

It is better to always have the error handling in the same way.

Fixes: ad4f15dc2c70 ("xen/netfront: don't bug in case of too many frags")
Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
drivers/net/xen-netfront.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 8d33970..5f5722b 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -906,7 +906,7 @@ static RING_IDX xennet_fill_frags(struct netfront_queue *queue,
__pskb_pull_tail(skb, pull_to - skb_headlen(skb));
if (unlikely(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS)) {
- queue->rx.rsp_cons = ++cons;
+ queue->rx.rsp_cons = ++cons + skb_queue_len(list);
return ~0U;