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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, Juergen Gross <jgross@suse.com>,
        "David S. Miller" <davem@davemloft.net>
Subject: [PATCH 4.18 44/88] xen/netfront: dont bug in case of too many frags
Date:   Thu, 27 Sep 2018 11:03:25 +0200
Message-Id: <20180927090306.164812304@linuxfoundation.org>
In-Reply-To: <20180927090300.631426620@linuxfoundation.org>
References: <20180927090300.631426620@linuxfoundation.org>
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4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Juergen Gross <jgross@suse.com>

commit ad4f15dc2c70b1de5e0a64d27335962fbc9cf71c upstream.

Commit 57f230ab04d291 ("xen/netfront: raise max number of slots in
xennet_get_responses()") raised the max number of allowed slots by one.
This seems to be problematic in some configurations with netback using
a larger MAX_SKB_FRAGS value (e.g. old Linux kernel with MAX_SKB_FRAGS
defined as 18 instead of nowadays 17).

Instead of BUG_ON() in this case just fall back to retransmission.

Fixes: 57f230ab04d291 ("xen/netfront: raise max number of slots in xennet_get_responses()")
Cc: stable@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/net/xen-netfront.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -907,7 +907,11 @@ static RING_IDX xennet_fill_frags(struct
 			BUG_ON(pull_to <= skb_headlen(skb));
 			__pskb_pull_tail(skb, pull_to - skb_headlen(skb));
 		}
-		BUG_ON(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS);
+		if (unlikely(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS)) {
+			queue->rx.rsp_cons = ++cons;
+			kfree_skb(nskb);
+			return ~0U;
+		}
 
 		skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,
 				skb_frag_page(nfrag),
@@ -1044,6 +1048,8 @@ err:
 		skb->len += rx->status;
 
 		i = xennet_fill_frags(queue, skb, &tmpq);
+		if (unlikely(i == ~0U))
+			goto err;
 
 		if (rx->flags & XEN_NETRXF_csum_blank)
 			skb->ip_summed = CHECKSUM_PARTIAL;