Re: [PATCH] scsi: virtio-scsi: Fix address translation failure of HighMem pages used by sg list

[Boaz Harrosh <bharrosh@panasas.com>]

On 07/25/2012 02:44 PM, Sen Wang wrote:

> 2012/7/25 Paolo Bonzini <pbonzini@redhat.com>:
>> Il 25/07/2012 10:29, Wang Sen ha scritto:
>>> 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 there are some HighMem pages in
>>> table->sgl 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.
>>
>> Heh, I was compiling (almost) the same patch as we speak. :)
> 
> Uh, what a coincidence! :)
> 
>>
>> I've never seen QEMU crash; the VM would more likely just fail to boot
>> with a panic.  But it's the same bug anyway.
> 
> I never met this before. How this situation happens?
> 
>>
>>> My solution is using sg_set_page instead of sg_set_buf.
>>>
>>> I have tested the patch on my workstation. QEMU would not crash any more.
>>>
>>> Signed-off-by: Wang Sen <senwang@linux.vnet.ibm.com>
>>> ---
>>>  drivers/scsi/virtio_scsi.c |    3 ++-
>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
>>> index 1b38431..fc5c88a 100644
>>> --- a/drivers/scsi/virtio_scsi.c
>>> +++ b/drivers/scsi/virtio_scsi.c
>>> @@ -198,7 +198,8 @@ static void virtscsi_map_sgl(struct scatterlist *sg, unsigned int *p_idx,
>>>       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_set_page(&sg[idx++], sg_page(sg_elem), sg_elem->length,
>>> +                     sg_elem->offset);
>>
>> This can simply be
>>
>>    sg[idx++] = *sg_elem;
>>
> 
> Yes, I saw your another E-mail. I think you're right. Simply calling
> sg_set_page can not handle
> the flag bits correctly. So, I'll repost the patch soon. Thank you!
> 


No this code is correct, though you will need to make sure to properly
terminate the destination sg_list.

But since old code was using sg_set_buf(), than it means it was properly
terminated before, and there for this code is good as is please don't
touch it.

Thanks
Boaz

>> Can you repost it with this change, and also add stable@vger.kernel.org
>> to the Cc?  Thanks very much!
>>
>> Paolo
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> 
> 
>