CVE-2026-89578: dm-io: clone the source bio instead of copying its biovec
In the Linux kernel, the following vulnerability has been resolved:
dm-io: clone the source bio instead of copying its biovec
For DMIOBIO requests, doregion() built each destination bio by walking the source bio's biovec and re-adding the pages one at a time, tracking the remaining transfer in sectors. The vector lengths are byte granular and need not be sector aligned (e.g. a misaligned ODIRECT buffer split across pages), so the sector-based accounting could lose a sub-sector fragment: tosector() truncated the remainder and the outer loop spun forever submitting empty bios, hanging the I/O.
There is no need to rebuild the biovec at all. The destination reads into (or writes from) exactly the same pages as the source bio, so the bio can simply clone the source's biovec with bioallocclone() and remap it to the target device. The clone inherits the source's iterator and alignment, and the block layer splits it to the target's limits on submission, so the whole region maps to a single cloned bio with no manual page copying or sector accounting.
This removes the per-page copy path (and its open-coded bvec dpages helpers) for bio-backed I/O and fixes the hang on misaligned direct I/O to a dm-mirror device. Page-list, vma and kmem sources keep the existing copy path.
Affected Software
Event History
Frequently Asked Questions
What workload is required to trigger the hang?
The affected path is DM_IO_BIO I/O with a source bio whose vector lengths are not sector-aligned. The description gives a misaligned O_DIRECT buffer split across pages as an example.
What is the practical impact if the issue is triggered?
Sector-based accounting can discard a sub-sector fragment, causing the code to repeatedly submit empty bios. This can hang the affected I/O operation.
Which systems are exposed?
Systems using the Linux kernel device-mapper dm-io bio-backed request path are relevant. Exposure specifically depends on issuing DM_IO_BIO requests with misaligned, non-sector-aligned bio vectors.
How can the issue be mitigated before applying a fix?
Avoid workloads that create misaligned direct I/O buffers split across pages in the affected DM_IO_BIO path. Keeping I/O buffers sector-aligned avoids the sub-sector accounting condition described.