CVE-2026-72108

NVD	https://nvd.nist.gov/vuln/detail/CVE-2026-72108
CWE	https://cwe.mitre.org/data/definitions/.html

In the Linux kernel, the following vulnerability has been resolved:

dm thin metadata: fix metadata snapshot consistency on commit failure

__reserve_metadata_snap() and __release_metadata_snap() modify the superblocks held_root directly in the block_managers buffer. If the subsequent metadata commit fails, the held_root gets flushed to disk through the abort_transaction path, resulting in inconsistent metadata.

Reproducer 1: __reserve_metadata_snap()

Create a 2 MiB metadata device and make the region after the 14th block inaccessible, to trigger metadata commit failure in the subsequent reserve_metadata_snap operation. The 14th block will be the shadow destination for the index block.

dmsetup create tmeta –table 0 112 linear /dev/sdc 0 112 3984 error

Create a 16 MiB thin-pool

dmsetup create tdata –table 0 32768 zero dd if=/dev/zero of=/dev/mapper/tmeta bs=4k count=1 dmsetup create tpool –table 0 32768 thin-pool /dev/mapper/tmeta /dev/mapper/tdata 128 0 1 skip_block_zeroing

Take a metadata snapshot to trigger metadata commit failure and transaction abort. However, the held_root is written to disk, breaking metadata consistency.

dmsetup message tpool 0 reserve_metadata_snap

thin_check v1.2.2 result:

Bad reference count for metadata block 6. Expected 2, but space map contains 1. Bad reference count for metadata block 7. Expected 2, but space map contains 1. Bad reference count for metadata block 13. Expected 1, but space map contains 0.

Reproducer 2: __release_metadata_snap()

Create a 2 MiB metadata device and make the region after the 16th block inaccessible, to trigger metadata commit failure in the subsequent release_metadata_snap operation. The 16th block will be the shadow destination for the index block.

dmsetup create tmeta –table 0 128 linear /dev/sdc 0 128 3968 error

Create a 16 MiB thin-pool

Reserve then release the metadata snapshot, to trigger metadata commit failure and transaction abort. The held_root gets removed from the on-disk superblock, causing inconsistent metadata.

dmsetup message tpool 0 reserve_metadata_snap dmsetup message tpool 0 release_metadata_snap

thin_check v1.2.2 result:

Bad reference count for metadata block 6. Expected 1, but space map contains 2. Bad reference count for metadata block 7. Expected 1, but space map contains 2. 1 metadata blocks have leaked.

Fix by deferring the held_root update to commit time.

Additionally, move the existing-snapshot check in __reserve_metadata_snap before the shadow operation to avoid unnecessary work. In __release_metadata_snap, clear pmd->held_root before btree deletion so partial failure leaks blocks rather than leaving a stale reference, and unlock the snapshot block before decrementing its refcount.

References