Linux Virtualization - Jan 2017 - [PATCH] virtio_blk: avoid DMA to stack for the sense buffer

Without this fix attempts to do scsi passthrough on virtio_blk will crash
the system on virtually mapped stacks, which is something happening during
boot with many distros.

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 at lst.de>
---
 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);
 		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

On 2017?01?04? 13:25, 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 at lst.de>
> ---
>   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);
>   		sgs[num_out + num_in++] = &sense;
>   		sg_init_one(&inhdr, &vbr->in_hdr, sizeof(vbr->in_hdr));
>   		sgs[num_out + num_in++] = &inhdr;
Acked-by: Jason Wang <jasowang at redhat.com>

Hi Christoph,

2017-01-04 6:25 GMT+01:00 Christoph Hellwig <hch at
lst.de>:
> 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.
This sounds scary.
Could you share how to reproduce it, this should go into stable if
it's the case.

Thanks,
Jinpu
>
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> ---
>  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);
>                 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
>

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 at lst.de>
> ---
>  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 at redhat.com>

>  		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