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

[Eric Blake]

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
>
> In the last step offset - zero_start causes an integer wraparound.
>
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
>   block/qcow2.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 2ba0b17c39..6d34d28c60 100644
> --- a/block/qcow2.c
> +++ 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);
> +

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);

pre-patch, it called zeroize(, 0x90000, 0xffffffffffffdc00, )
post-patch, it calls zeroize(, 0x8dc00, 0, )

Looking at qcow2_cluster_zeroize, we have:
    assert(QEMU_IS_ALIGNED(offset, s->cluster_size));

which will now trigger.  This patch is a good idea, but needs a v2.

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