[PATCH 3.16 112/132] vhost: make sure log_num < in_num

[Ben Hutchings]

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

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

From: yongduan <yongduan@xxxxxxxxxxx>

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@xxxxxxxxxxx>
Signed-off-by: ruippan <ruippan@xxxxxxxxxxx>
Signed-off-by: yongduan <yongduan@xxxxxxxxxxx>
Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Reviewed-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 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;