From: Eric Blake <eblake@redhat.com>
To: Alberto Garcia <berto@igalia.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>,
qemu-block@nongnu.org, Max Reitz <mreitz@redhat.com>
Subject: Re: [PATCH] qcow2: Avoid integer wraparound in qcow2_co_truncate()
Date: Fri, 1 May 2020 12:12:42 -0500 [thread overview]
Message-ID: <5ba91898-9d3b-d55d-c360-83cca41d88f4@redhat.com> (raw)
In-Reply-To: <20200501131525.6745-1-berto@igalia.com>
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 <berto@igalia.com>
> ---
> 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
next prev parent reply other threads:[~2020-05-01 17:13 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-01 13:15 [PATCH] qcow2: Avoid integer wraparound in qcow2_co_truncate() Alberto Garcia
2020-05-01 17:12 ` Eric Blake [this message]
2020-05-01 18:48 ` Eric Blake
2020-05-04 13:47 ` Alberto Garcia
2020-05-04 14:51 ` Eric Blake
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=5ba91898-9d3b-d55d-c360-83cca41d88f4@redhat.com \
--to=eblake@redhat.com \
--cc=berto@igalia.com \
--cc=kwolf@redhat.com \
--cc=mreitz@redhat.com \
--cc=qemu-block@nongnu.org \
--cc=qemu-devel@nongnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).