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

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@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;