qemu-img: Deprecate use of -b without -F

Creating an image that requires format probing of the backing image is
potentially 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).  For example, 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.
Fortunately, the worst effects occur only when the backing image is
originally raw, and we at least prevent commit into a probed raw
backing file that would change its probed type.

Still, it is worth starting a deprecation clock so that future
qemu-img can refuse to create backing chains that would rely on
probing, to encourage clients to avoid unsafe practices.  Most
warnings are intentionally emitted from bdrv_img_create() in the block
layer, but qemu-img convert uses bdrv_create() which cannot emit its
own warning without causing spurious warnings on other code paths.  In
the end, all command-line image creation or backing file rewriting now
performs a check.

Furthermore, if we probe a backing file as non-raw, then it is safe to
explicitly record that result (rather than relying on future probes);
only where we probe a raw image do we care about further warnings to
the user when using such an image (for example, commits into a
probed-raw backing file are prevented), to help them improve their
tooling.  But whether or not we make the probe results explicit, we
still warn the user to remind them to upgrade their workflow to supply
-F always.

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 301
also shows a change to qcow messages.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200706203954.341758-11-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

docs/system/deprecated.rst

@@ -439,6 +439,26 @@ image).  Rather, any changes to the backing chain should be performed
 with ``qemu-img rebase -u`` either before or after the remaining
 changes being performed by amend, as appropriate.
 
+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
 -----------------------