Re: [PATCH] virtio_blk: avoid DMA to stack for the sense buffer

[Michael S. Tsirkin]

On Wed, Jan 04, 2017 at 08:25:05AM +0300, Christoph Hellwig wrote:
> Most users of BLOCK_PC requests allocate the sense buffer on the stack,
> so to avoid DMA to the stack copy them to a field in the heap allocated
> virtblk_req structure.  Without that any attempt at SCSI passthrough I/O,
> including the SG_IO ioctl from userspace will crash the kernel.  Note that
> this includes running tools like hdparm even when the host does not have
> SCSI passthrough enabled.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  drivers/block/virtio_blk.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 5545a67..3c3b8f6 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -56,6 +56,7 @@ struct virtblk_req {
>  	struct virtio_blk_outhdr out_hdr;
>  	struct virtio_scsi_inhdr in_hdr;
>  	u8 status;
> +	u8 sense[SCSI_SENSE_BUFFERSIZE];
>  	struct scatterlist sg[];
>  };
>  
> @@ -102,7 +103,8 @@ static int __virtblk_add_req(struct virtqueue *vq,
>  	}
>  
>  	if (type == cpu_to_virtio32(vq->vdev, VIRTIO_BLK_T_SCSI_CMD)) {
> -		sg_init_one(&sense, vbr->req->sense, SCSI_SENSE_BUFFERSIZE);
> +		memcpy(vbr->sense, vbr->req->sense, SCSI_SENSE_BUFFERSIZE);
> +		sg_init_one(&sense, vbr->sense, SCSI_SENSE_BUFFERSIZE);

I would prefer sizeof(vbr->sense) here to avoid duplication.
Otherwise:

Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>


>  		sgs[num_out + num_in++] = &sense;
>  		sg_init_one(&inhdr, &vbr->in_hdr, sizeof(vbr->in_hdr));
>  		sgs[num_out + num_in++] = &inhdr;
> -- 
> 2.1.4