[PATCH 3.13.y-ckt 060/156] macvtap: make sure neighbour code can push ethernet header

[Kamal Mostafa]

3.13.11-ckt19 -stable review patch.  If anyone has any objections, please let me know.

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

From: Eric Dumazet <edumazet@xxxxxxxxxx>

[ Upstream commit 2f1d8b9e8afa5a833d96afcd23abcb8cdf8d83ab ]

Brian reported crashes using IPv6 traffic with macvtap/veth combo.

I tracked the crashes in neigh_hh_output()

-> memcpy(skb->data - HH_DATA_MOD, hh->hh_data, HH_DATA_MOD);

Neighbour code assumes headroom to push Ethernet header is
at least 16 bytes.

It appears macvtap has only 14 bytes available on arches
where NET_IP_ALIGN is 0 (like x86)

Effect is a corruption of 2 bytes right before skb->head,
and possible crashes if accessing non existing memory.

This fix should also increase IPv4 performance, as paranoid code
in ip_finish_output2() wont have to call skb_realloc_headroom()

Reported-by: Brian Rak <brak@xxxxxxxxx>
Tested-by: Brian Rak <brak@xxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
---
 drivers/net/macvtap.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
index 5aead8c..d1c23e6 100644
--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c
@@ -635,12 +635,15 @@ static int macvtap_skb_to_vnet_hdr(const struct sk_buff *skb,
 	return 0;
 }
 
+/* Neighbour code has some assumptions on HH_DATA_MOD alignment */
+#define MACVTAP_RESERVE HH_DATA_OFF(ETH_HLEN)
+
 /* Get packet from user space buffer */
 static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
 				const struct iovec *iv, unsigned long total_len,
 				size_t count, int noblock)
 {
-	int good_linear = SKB_MAX_HEAD(NET_IP_ALIGN);
+	int good_linear = SKB_MAX_HEAD(MACVTAP_RESERVE);
 	struct sk_buff *skb;
 	struct macvlan_dev *vlan;
 	unsigned long len = total_len;
@@ -699,7 +702,7 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
 			linear = vnet_hdr.hdr_len;
 	}
 
-	skb = macvtap_alloc_skb(&q->sk, NET_IP_ALIGN, copylen,
+	skb = macvtap_alloc_skb(&q->sk, MACVTAP_RESERVE, copylen,
 				linear, noblock, &err);
 	if (!skb)
 		goto err;
-- 
1.9.1

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