linux-kernel - [PATCH 3.16 112/132] vhost: make sure log_num < in

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Message-ID: <lsq.1568989415.700934050@decadent.org.uk>
Date:   Fri, 20 Sep 2019 15:23:35 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "ruippan" <ruippan@...cent.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "yongduan" <yongduan@...cent.com>,
        "Tyler Hicks" <tyhicks@...onical.com>,
        "Lidong Chen" <lidongchen@...cent.com>
Subject: [PATCH 3.16 112/132] vhost: make sure log_num < in_num

3.16.74-rc1 review patch.  If anyone has any objections, please let me know.

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

From: yongduan <yongduan@...cent.com>

commit 060423bfdee3f8bc6e2c1bac97de24d5415e2bc4 upstream.

The code assumes log_num < in_num everywhere, and that is true as long as
in_num is incremented by descriptor iov count, and log_num by 1. However
this breaks if there's a zero sized descriptor.

As a result, if a malicious guest creates a vring desc with desc.len = 0,
it may cause the host kernel to crash by overflowing the log array. This
bug can be triggered during the VM migration.

There's no need to log when desc.len = 0, so just don't increment log_num
in this case.

Fixes: 3a4d5c94e959 ("vhost_net: a kernel-level virtio server")
Reviewed-by: Lidong Chen <lidongchen@...cent.com>
Signed-off-by: ruippan <ruippan@...cent.com>
Signed-off-by: yongduan <yongduan@...cent.com>
Acked-by: Michael S. Tsirkin <mst@...hat.com>
Reviewed-by: Tyler Hicks <tyhicks@...onical.com>
Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/vhost/vhost.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1194,7 +1194,7 @@ static int get_indirect(struct vhost_vir
 		/* If this is an input descriptor, increment that count. */
 		if (desc.flags & VRING_DESC_F_WRITE) {
 			*in_num += ret;
-			if (unlikely(log)) {
+			if (unlikely(log && ret)) {
 				log[*log_num].addr = desc.addr;
 				log[*log_num].len = desc.len;
 				++*log_num;
@@ -1317,7 +1317,7 @@ int vhost_get_vq_desc(struct vhost_virtq
 			/* If this is an input descriptor,
 			 * increment that count. */
 			*in_num += ret;
-			if (unlikely(log)) {
+			if (unlikely(log && ret)) {
 				log[*log_num].addr = desc.addr;
 				log[*log_num].len = desc.len;
 				++*log_num;