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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Dongli Zhang <dongli.zhang@oracle.com>,
        "David S. Miller" <davem@davemloft.net>
Subject: [PATCH 4.9 39/74] xen-netfront: do not assume sk_buff_head list is empty in error handling
Date:   Fri, 20 Sep 2019 00:03:52 +0200
Message-Id: <20190919214808.712612375@linuxfoundation.org>
In-Reply-To: <20190919214800.519074117@linuxfoundation.org>
References: <20190919214800.519074117@linuxfoundation.org>
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From: Dongli Zhang <dongli.zhang@oracle.com>

[ Upstream commit 00b368502d18f790ab715e055869fd4bb7484a9b ]

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@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/net/xen-netfront.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -907,7 +907,7 @@ static RING_IDX xennet_fill_frags(struct
 			__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);
 			kfree_skb(nskb);
 			return ~0U;
 		}