0009-xen-netfront-do-not-use-data-already-exposed-to-back.patch

From f2314a5da6c83648d494844f91014a2d6d1ab31d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Marczykowski-G=C3=B3recki?=
 <marmarek@invisiblethingslab.com>
Date: Wed, 16 Dec 2015 05:19:37 +0100
Subject: [PATCH] xen-netfront: do not use data already exposed to backend
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Backend may freely modify anything on shared page, so use data which was
supposed to be written there, instead of reading it back from the shared
page.

This is part of XSA155.

CC: stable@vger.kernel.org
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
---
 drivers/net/xen-netfront.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index abb9b3cd87b8..56c8a4a32672 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -456,7 +456,7 @@ static void xennet_tx_setup_grant(unsigned long gfn, unsigned int offset,
 	tx->flags = 0;
 
 	info->tx = tx;
-	info->size += tx->size;
+	info->size += len;
 }
 
 static struct xen_netif_tx_request *xennet_make_first_txreq(
@@ -572,7 +572,7 @@ static netdev_tx_t xennet_start_xmit(struct sk_buff *skb, struct net_device *dev
 	int slots;
 	struct page *page;
 	unsigned int offset;
-	unsigned int len;
+	unsigned int len, this_len;
 	unsigned long flags;
 	struct netfront_queue *queue = NULL;
 	unsigned int num_queues = dev->real_num_tx_queues;
@@ -632,14 +632,15 @@ static netdev_tx_t xennet_start_xmit(struct sk_buff *skb, struct net_device *dev
 	}
 
 	/* First request for the linear area. */
+	this_len = min_t(unsigned int, XEN_PAGE_SIZE - offset, len);
 	first_tx = tx = xennet_make_first_txreq(queue, skb,
 						page, offset, len);
-	offset += tx->size;
+	offset += this_len;
 	if (offset == PAGE_SIZE) {
 		page++;
 		offset = 0;
 	}
-	len -= tx->size;
+	len -= this_len;
 
 	if (skb->ip_summed == CHECKSUM_PARTIAL)
 		/* local packet? */
-- 
2.25.3