CVE-2026-46107: dm-thin: fix metadata refcount underflow

Severity: high (CVSS 7.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-28 · updated 2026-10-19
Weakness: CWE-191

Description

In the Linux kernel, the following vulnerability has been resolved: dm-thin: fix metadata refcount underflow There's a bug in dm-thin in the function rebalance_children. If the internal btree node has one entry, the code tries to copy all btree entries from the node's child to the node itself and then decrement the child's reference count. If the child node is shared (it has reference count > 1), we won't free it, so there would be two pointers to each of the grandchildren nodes. But the reference counts of the grandchildren is not increased, thus the reference count doesn't match the number of pointers that point to the grandchildren. This results in "device mapper: space map common: unable to decrement block" errors. Fix this bug by incrementing reference counts on the grandchildren if the btree node is shared.

Affected products

Linux kernel: from 3.2 before 5.10.259; from 5.11 before 5.15.209; from 5.16 before 6.1.175; from 6.2 before 6.6.140; from 6.7 before 6.12.88; from 6.13 before 6.18.30 — fixed in 5.10.259, 5.15.209, 6.1.175, 6.12.88, 6.18.30, 6.6.140

How to fix

Upgrade to a fixed version: Linux kernel 5.10.259, 5.15.209, 6.1.175, 6.12.88. Check the vendor advisory for mitigations.

References

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