On a Thursday in 2025, Peter Krempa via Devel wrote:
From: Peter Krempa <pkre...@redhat.com>There are 3 bugs in the qcow2 header extension parser: 1) Header extension padding not taken into account Per qcow2 documentation (qemu.git/docs/interop/qcow2.txt, also now mirrored in the comment explaining the parser) each header extension entry is padded to a multiple of 8 bytes. Our parser didn't take the padding into account and advanced the current offset only by the 'length', which corresponds to the length of the data. This meant that in vast majority of cases only the first extension would be parsed correctly. For any other one we'd try to fetch the magic and length from wrong place. Luckily this wasn't problem for most of the almost 15 years this bug
a problem
existed as we only cared about the backing file format string header which was always stored first by qemu. It is now a problem in the very specific case when a qcow2 image has a 'data-file' and also a backing store with format. In such case we'd parse the backing store format properly as it's the first header and 'data-file' being the second would be skipped. The buffer bounds checks were correct so we didn't violate any memory boundaries. 2) Integer underflow in calculation of end of header extension block If the image doesn't have a backing image, the 'backing_file_offset' qcow2 header field is 0. We use that value as 'extensions_end' which is used in the while loop to parse the extension entries. The check was done as "offset < (extensions_end - 8)", thus it unreflows when there's no filename. The design of the loop prevented anything bad from happening though. 3) Off-by-one when determining end of header extension block The aforementioned end of extension check above also has an off-by-one error as it allowed another loop if more than 8 bytes were available. Now the termination entry has data length of 0 bytes so we'd not be able to properly process that one. This wasn't a problem either as for now there's just the terminator having 0 byte lenght.
length
This patch improves documentation by quoting the qcow2 interoperability document and adjusts the loop condition and lenght handling to comply with the specs.
length
Interestingly we also had a test case for this specific scenario but the expected test output was wrong. Fixes: a93402d48b2996e5300754d299ef0d3f646aa098 Resolves: https://issues.redhat.com/browse/RHEL-93775 Signed-off-by: Peter Krempa <pkre...@redhat.com> --- src/storage_file/storage_file_probe.c | 62 +++++++++---------- .../out/qcow2datafile-qcow2_qcow2-datafile | 10 ++- 2 files changed, 40 insertions(+), 32 deletions(-)
Reviewed-by: Ján Tomko <jto...@redhat.com> Jano
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