Re: [PATCH] qcow2: Avoid integer wraparound in qcow2_co_truncate()

[Eric Blake <eblake@redhat.com>]

On 5/1/20 12:12 PM, Eric Blake wrote:
> On 5/1/20 8:15 AM, Alberto Garcia wrote:
>> After commit f01643fb8b47e8a70c04bbf45e0f12a9e5bc54de when an image is
>> extended and BDRV_REQ_ZERO_WRITE is set then the new clusters are
>> zeroized.
>>
>> The code however does not detect correctly situations when the old and
>> the new end of the image are within the same cluster. The problem can
>> be reproduced with these steps:
>>
>>     qemu-img create -f qcow2 backing.qcow2 1M
>>     qemu-img create -f qcow2 -b backing.qcow2 top.qcow2
> 
> We should get in the habit of documenting -F qcow2 (I have a series, 
> still awaiting review, that would warn if you don't).
> 
>>     qemu-img resize --shrink top.qcow2 520k
>>     qemu-img resize top.qcow2 567k
>>

Since your reproducer triggers assertion failure, I suggest doing this 
instead:


>> +++ b/block/qcow2.c
>> @@ -4234,6 +4234,9 @@ static int coroutine_fn 
>> qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
>>       if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) {
>>           uint64_t zero_start = QEMU_ALIGN_UP(old_length, 
>> s->cluster_size);
>> +        /* zero_start should not be after the new end of the image */
>> +        zero_start = MIN(zero_start, offset);
>> +

Drop this hunk (leave zero_start unchanged), and instead...

> 
> So, using your numbers, pre-patch, we have zero_start = 0x90000 (0x82000 
> rounded up to 0x10000 alignment).  post-patch, the new MIN() lowers it 
> back to 0x8dc00 (the new size), which is unaligned.
> 
>>           /*
>>            * Use zero clusters as much as we can. qcow2_cluster_zeroize()
>>            * requires a cluster-aligned start. The end may be 
>> unaligned if it is
>           * at the end of the image (which it is here).
>           */
>          ret = qcow2_cluster_zeroize(bs, zero_start, offset - 
> zero_start, 0);

...patch _this_ call to compute 'QEMU_ALIGN_UP(offset, s->cluster_size) 
- zero_start' for the length.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org