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

[Michael S. Tsirkin]

On Thu, Jan 19, 2023 at 03:57:21PM +0200, Alexander Shishkin wrote:
> 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@xxxxxxxxxxxxxxx>
> ---
>  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;
> -- 
> 2.39.0