[PATCH v1 6/6] virtio_ring: Prevent bounds check bypass on descriptor index

[Alexander Shishkin <alexander.shishkin@linux.intel.com>]

The descriptor index in virtqueue_get_buf_ctx_split() comes from the
device/VMM.a Use array_index_nospec() to prevent the CPU from speculating
beyond the descriptor array bounds and providing a primitive for building
a side channel.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
---
 drivers/virtio/virtio_ring.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 2e7689bb933b..c42d070ab68d 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -9,6 +9,7 @@
 #include <linux/device.h>
 #include <linux/slab.h>
 #include <linux/module.h>
+#include <linux/nospec.h>
 #include <linux/hrtimer.h>
 #include <linux/dma-mapping.h>
 #include <linux/kmsan.h>
@@ -819,6 +820,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
 		BAD_RING(vq, "id %u out of range\n", i);
 		return NULL;
 	}
+
+	i = array_index_nospec(i, vq->split.vring.num);
 	if (unlikely(!vq->split.desc_state[i].data)) {
 		BAD_RING(vq, "id %u is not a head!\n", i);
 		return NULL;
-- 
2.39.0