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

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 04:47:03PM +0100, ??? wrote:
> This sounds scary.
> Could you share how to reproduce it, this should go into stable if
> it's the case.
Step 1: Build your kernel with CONFIG_VMAP_STACK=y
Step 2: issue a SG_IO ioctl, e.g. sg_inq /dev/vda

2017-01-05 10:57 GMT+01:00 Christoph Hellwig <hch at
lst.de>:
> On Wed, Jan 04, 2017 at 04:47:03PM +0100, ??? wrote:
>> This sounds scary.
>> Could you share how to reproduce it, this should go into stable if
>> it's the case.
>
> Step 1: Build your kernel with CONFIG_VMAP_STACK=y
> Step 2: issue a SG_IO ioctl, e.g. sg_inq /dev/vda
>
Thanks, so it's only relevant to kernel > 4.9, as  CONFIG_VMAP_STACK
only introduced in 4.9 kernel.

Regards,
Jinpu