linux-kernel - [ 099/218] SCSI: scsi: virtio-scsi: Fix address translation failure of HighMem pages used by sg list

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Date:	Fri, 28 Sep 2012 13:15:16 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	alan@...rguk.ukuu.org.uk, Wang Sen <senwang@...ux.vnet.ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	James Bottomley <JBottomley@...allels.com>
Subject: [ 099/218] SCSI: scsi: virtio-scsi: Fix address translation failure of HighMem pages used by sg list

3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Wang Sen <senwang@...ux.vnet.ibm.com>

commit 27e99ade81368e6fdda3212bff9345177cf9e57a upstream.

When using the commands below to write some data to a virtio-scsi LUN of the
QEMU guest(32-bit) with 1G physical memory(qemu -m 1024), the qemu will crash.

        # sudo mkfs.ext4 /dev/sdb  (/dev/sdb is the virtio-scsi LUN.)
        # sudo mount /dev/sdb /mnt
        # dd if=/dev/zero of=/mnt/file bs=1M count=1024

In current implementation, sg_set_buf is called to add buffers to sg list which
is put into the virtqueue eventually. But if there are some HighMem pages in
table->sgl you can not get virtual address by sg_virt. So, sg_virt(sg_elem) may
return NULL value. This will cause QEMU exit when virtqueue_map_sg is called
in QEMU because an invalid GPA is passed by virtqueue.

Two solutions are discussed here:
http://lkml.indiana.edu/hypermail/linux/kernel/1207.3/00675.html

Finally, value assignment approach was adopted because:

Value assignment creates a well-formed scatterlist, because the termination
marker in source sg_list has been set in blk_rq_map_sg(). The last entry of the
source sg_list is just copied to the the last entry in destination list.  Note
that, for now, virtio_ring does not care about the form of the scatterlist and
simply processes the first out_num + in_num consecutive elements of the sg[]
array.

I have tested the patch on my workstation. QEMU would not crash any more.

Signed-off-by: Wang Sen <senwang@...ux.vnet.ibm.com>
Acked-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: James Bottomley <JBottomley@...allels.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/scsi/virtio_scsi.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -198,7 +198,7 @@ static void virtscsi_map_sgl(struct scat
 	int i;

 	for_each_sg(table->sgl, sg_elem, table->nents, i)
-		sg_set_buf(&sg[idx++], sg_virt(sg_elem), sg_elem->length);
+		sg[idx++] = *sg_elem;

 	*p_idx = idx;
 }

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