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

From: Boaz Harrosh <bharrosh@panasas.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Wang Sen <senwang@linux.vnet.ibm.com>,
	<linux-scsi@vger.kernel.org>, <JBottomley@parallels.com>,
	<stefanha@linux.vnet.ibm.com>, <mc@linux.vnet.ibm.com>,
	<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] scsi: virtio-scsi: Fix address translation failure of HighMem pages used by sg list
Date: Wed, 25 Jul 2012 12:22:48 +0300	[thread overview]
Message-ID: <500FBAE8.2050107@panasas.com> (raw)
In-Reply-To: <500FB1DE.1000100@redhat.com>

On 07/25/2012 11:44 AM, Paolo Bonzini wrote:

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

No! Please use sg_set_page()! Look at sg_set_page(), which calls sg_assign_page().
It has all these jump over chained arrays. When you'll start using long
sg_lists (which you should) then jumping from chain to chain must go through
sg_page(sg_elem) && sg_assign_page(), As in the original patch.

Thanks
Boaz

> Paolo
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