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

[Eric Blake]

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