CVE-2026-72108 — dm thin metadata: fix metadata snapshot consistency on commit failure

Affected: Linux

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 superblock's held_root directly in the block_manager's 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()

1. 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"

2. 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"

3. 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 c…

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CVE-2026-72108 — dm thin metadata: fix metadata snapshot consistency on commit failure