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From 92504013c1489168e3514f3c7342c016156c6af3 Mon Sep 17 00:00:00 2001
From: Michael S. Tsirkin <mst@redhat.com>
Date: Wed, 19 Mar 2014 17:21:02 +0200
Subject: [PATCH] virtio-net: fix guest-triggerable buffer overrun

RH-Author: Michael S. Tsirkin <mst@redhat.com>
Message-id: <1395242197-28749-1-git-send-email-mst@redhat.com>
O-Subject: [virt-devel][PATCH EMBARGOED qemu-kvm RHEL7.0] virtio-net: fix
Bugzilla: 1078308
RH-Acked-by: Markus Armbruster <armbruz@redhat.com>
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Acked-by: Vlad Yasevich <vyasevic@redhat.com>
RH-Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

When VM guest programs multicast addresses for
a virtio net card, it supplies a 32 bit
entries counter for the number of addresses.
These addresses are read into tail portion of
a fixed macs array which has size MAC_TABLE_ENTRIES,
at offset equal to in_use.

To avoid overflow of this array by guest, qemu attempts
to test the size as follows:
-    if (in_use + mac_data.entries <= MAC_TABLE_ENTRIES) {

however, as mac_data.entries is uint32_t, this sum
can overflow, e.g. if in_use is 1 and mac_data.entries
is 0xffffffff then in_use + mac_data.entries will be 0.

Qemu will then read guest supplied buffer into this
memory, overflowing buffer on heap.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1078308
Upstream status: EMBARGOED RT 284849
Brew build: http://brewweb.devel.redhat.com/brew/taskinfo?taskID=7225956
---
 hw/net/virtio-net.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 06c53fe..2d559e0 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -594,7 +594,7 @@ static int virtio_net_handle_mac(VirtIONet *n, uint8_t cmd,
         goto error;
     }
 
-    if (in_use + mac_data.entries <= MAC_TABLE_ENTRIES) {
+    if (mac_data.entries <= MAC_TABLE_ENTRIES - in_use) {
         s = iov_to_buf(iov, iov_cnt, 0, &macs[in_use * ETH_ALEN],
                        mac_data.entries * ETH_ALEN);
         if (s != mac_data.entries * ETH_ALEN) {
-- 
1.7.1