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qemu-img: Require -F with -b backing image

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Back in commit d9f059aa6c (qemu-img: Deprecate use of -b without -F),
we deprecated the ability to create a file with a backing image that
requires qemu to perform format probing.  Qemu can still probe older
files for backwards compatibility, but it is time to finish off the
ability to create such images, due to the potential security risk they
present.  Update a couple of iotests affected by the change.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210503213600.569128-3-eblake@redhat.com>
Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Eric Blake 2021-05-03 14:36:00 -07:00 • committed by Kevin Wolf
parent 5a385bf5c5
commit 497a30dbb0
10 changed files with 56 additions and 85 deletions
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20
docs/system/deprecated.rst
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@ -282,26 +282,6 @@ this CPU is also deprecated.
Related binaries
----------------
qemu-img backing file without format (since 5.1)
''''''''''''''''''''''''''''''''''''''''''''''''
The use of ``qemu-img create``, ``qemu-img rebase``, or ``qemu-img
convert`` to create or modify an image that depends on a backing file
now recommends that an explicit backing format be provided. This is
for safety: if QEMU probes a different format than what you thought,
the data presented to the guest will be corrupt; similarly, presenting
a raw image to a guest allows a potential security exploit if a future
probe sees a non-raw image based on guest writes.
To avoid the warning message, or even future refusal to create an
unsafe image, you must pass ``-o backing_fmt=`` (or the shorthand
``-F`` during create) to specify the intended backing format. You may
use ``qemu-img rebase -u`` to retroactively add a backing format to an
existing image. However, be aware that there are already potential
security risks to blindly using ``qemu-img info`` to probe the format
of an untrusted backing image, when deciding what format to add into
an existing image.
Backwards compatibility
-----------------------