Re: [PATCH v1 6/6] virtio_ring: Prevent bounds check bypass on descriptor index
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2023-01-20 12:57:38
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On Thu, Jan 19, 2023 at 03:57:21PM +0200, Alexander Shishkin wrote:
quoted hunk
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);
I suspect plain i &= split.vring.num - 1 is more efficient. We know num is a power of two but compiler doesn't. And pls add a comment explaining what's going on.
if (unlikely(!vq->split.desc_state[i].data)) {
BAD_RING(vq, "id %u is not a head!\n", i);
return NULL;
--
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