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 <eblake@redhat.com> --- 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 v4.2.0-2399-g3cba0d19f2 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 refcount_bits=16 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: <2fd580c2-4b94-4430-1072-ef04bbd2da60@redhat.com> For completeness' sake: $> ~/build/v4_tightened_qemu-img-QEMU/qemu-img info --backing-chain flattened.qcow2 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 [...] -- /kashyap