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Re: [PATCH v4 7/7] qemu-img: Deprecate use of -b without -F

From: Kashyap Chamarthy
Subject: Re: [PATCH v4 7/7] qemu-img: Deprecate use of -b without -F
Date: Tue, 17 Mar 2020 17:21:46 +0100

On Thu, Mar 12, 2020 at 02:28:22PM -0500, Eric Blake wrote:
> Creating an image that requires format probing of the backing image is
> inherently unsafe (we've had several CVEs over the years based on
> probes leaking information to the guest on a subsequent boot, although
> these days tools like libvirt are aware of the issue enough to prevent
> the worst effects).  However, if our probing algorithm ever changes,
> or if other tools like libvirt determine a different probe result than
> we do, then subsequent use of that backing file under a different
> format will present corrupted data to the guest.  Start a deprecation
> clock so that future qemu-img can refuse to create unsafe backing
> chains that would rely on probing.  The warnings are intentionally
> emitted from the block layer rather than qemu-img (thus, all paths
> into image creation or rewriting perform the check).
> However, there is one time where probing is safe: if we probe raw,
> then it is safe to record that implicitly in the image (but we still
> warn, as it's better to teach the user to supply -F always than to
> make them guess when it is safe).
> iotest 114 specifically wants to create an unsafe image for later
> amendment rather than defaulting to our new default of recording a
> probed format, so it needs an update.  While touching it, expand it to
> cover all of the various warnings enabled by this patch.  iotest 290
> also shows a change to qcow messages; note that the fact that we now
> make a probed format of 'raw' explicit now results in a double
> warning, but no one should be creating new qcow images so it is not
> worth cleaning up.
> Signed-off-by: Eric Blake <address@hidden>
> ---
>  docs/system/deprecated.rst | 19 +++++++++++++++++++
>  block.c                    | 21 ++++++++++++++++++++-
>  qemu-img.c                 |  2 +-
>  tests/qemu-iotests/114     | 11 +++++++++++
>  tests/qemu-iotests/114.out |  8 ++++++++
>  tests/qemu-iotests/290.out |  5 ++++-
>  6 files changed, 63 insertions(+), 3 deletions(-)

[A quick question ... while I'm still testing]

I just applied your v4, and I'm here:

    $> git describe

Expected warning on 'create' wiht no -F:

    $> ~/build/v4_tightened_qemu-img-QEMU/qemu-img create -f qcow2 -b 
./base.raw ./overlay1.qcow2
    qemu-img: warning: Deprecated use of backing file without explicit backing 
format (detected format of raw)
    Formatting './overlay1.qcow2', fmt=qcow2 size=4294967296 
backing_file=./base.raw backing_fmt=raw cluster_size=65536 lazy_refcounts=off 

But here is the lack of warning with 'convert' expected?

    $> ~/build/v4_tightened_qemu-img-QEMU/qemu-img convert -B ./base.raw -O 
qcow2 overlay1.qcow2 flattened.qcow2

In your response on the v3, you said the above should throw a warning;
refer to Message-ID: <address@hidden>

For completeness' sake:

    $> ~/build/v4_tightened_qemu-img-QEMU/qemu-img info --backing-chain 
    image: flattened.qcow2
    file format: qcow2
    virtual size: 4 GiB (4294967296 bytes)
    disk size: 196 KiB
    cluster_size: 65536
    backing file: ./base.raw
    Format specific information:
        compat: 1.1
        lazy refcounts: false
        refcount bits: 16
        corrupt: false
    image: ./base.raw
    file format: raw
    virtual size: 4 GiB (4294967296 bytes)
    disk size: 778 MiB



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