Junglewise Threat Intelligence

CVE-2026-45920: Linux Kernel ext4 double decrement in mballoc during shutdown

CVE-2026-45920 · Severity: info · CVSS 0 · Published 2026-05-27

Technologies: Linux Kernel. Vendors: Linux.

Executive brief

A vulnerability was identified in the Linux kernel's ext4 filesystem driver where certain error conditions during a filesystem shutdown could cause internal block counters to become inconsistent. This issue primarily manifests as system warnings or potential stability issues when unmounting a filesystem that has experienced unexpected shutdowns or heavy stress. While it does not directly expose user data, it represents a flaw in how the system manages disk space reservations during failure scenarios.

Technical details

A logic error in the ext4 filesystem driver's multi-block allocator (mballoc) caused a double decrement of the 's_dirtyclusters_counter' during filesystem shutdown or error injection tests. The issue occurred in the error path between 'ext4_mb_mark_diskspace_used()' and its caller 'ext4_mb_new_blocks()'. When 'ext4_handle_dirty_metadata()' returned an error due to a shutdown, 'ext4_mb_mark_diskspace_used()' would decrement the cluster counters before returning the error; the caller would then fall into an error path that decremented the same counter a second time. This resulted in the counter underflowing to -1, triggering a kernel warning in 'ext4_put_super()'. The fix consolidates the counter reduction logic into a single location in the caller to ensure consistent ownership of cluster reservations.

Affected products

  • Linux Linux kernel ext4 filesystem driver

Timeline

  • 2026-05-27: disclosed: Initial disclosure and patch availability via kernel.org
  • 2026-05-27: advisory: NVD publication of CVE-2026-45920

References

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