CVE-2026-68442 in Linux
Summary
by MITRE • 08/12/2026
In the Linux kernel, the following vulnerability has been resolved:
btrfs: don't propagate EXTENT_FLAG_LOGGING to split extent maps
When btrfs_drop_extent_map_range() splits an extent map, the new split maps inherit the original map's flags through a local 'flags' variable. Commit f86f7a75e2fb ("btrfs: use the flags of an extent map to identify the compression type") changed the EXTENT_FLAG_LOGGING clearing to operate on em->flags instead of that local 'flags' copy, so a split of an extent map that is currently being logged wrongly inherits EXTENT_FLAG_LOGGING.
The flag is then never cleared on the split, and when it is freed while still on the inode's modified_extents list (for example by the extent map shrinker) it trips the WARN_ON(!list_empty(&em->list)) in btrfs_free_extent_map() and leads to a use-after-free.
Clear EXTENT_FLAG_LOGGING from the local 'flags' copy used for the splits and only clear EXTENT_FLAG_PINNED from em->flags, restoring the behaviour prior to f86f7a75e2fb.
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Analysis
by VulDB Data Team • 08/12/2026
The vulnerability in the Linux kernel's btrfs filesystem implementation stems from an improper handling of extent map flags during split operations, specifically involving the EXTENT_FLAG_LOGGING flag. This issue manifests when btrfs_drop_extent_map_range() function processes extent map splitting, where the newly created split maps inherit flags from the original map through a local 'flags' variable rather than properly managing the flag state. The problem was introduced by commit f86f7a75e2fb which altered how EXTENT_FLAG_LOGGING clearing operates, shifting from the local 'flags' copy to direct modification of em->flags. This change created a scenario where extent maps currently being logged incorrectly propagate the logging flag to their split children, causing subsequent operational failures.
The technical flaw creates a dangerous state where the EXTENT_FLAG_LOGGING flag persists in split extent maps even after the original map should have cleared it. When these improperly flagged extent maps are eventually freed while still present on an inode's modified_extents list, typically through the extent map shrinker mechanism, they trigger a critical warning condition in btrfs_free_extent_map() function. This warning specifically checks for empty lists with the WARN_ON(!list_empty(&em->list)) assertion, which fails when the split extent maps retain the logging flag and their associated memory management structures remain improperly linked. The consequence of this failure is a use-after-free vulnerability that can lead to system instability and potential exploitation.
The operational impact of this vulnerability extends beyond simple memory corruption, as it represents a fundamental flaw in btrfs's extent map management and memory cleanup processes. When the kernel attempts to free extent maps that have retained logging flags, the improper list state causes the system to detect an inconsistency that should never occur under normal operation. This scenario can lead to kernel panics, system crashes, or more insidiously, provide opportunities for privilege escalation attacks through controlled memory corruption. The vulnerability affects systems running btrfs filesystems where extensive extent map operations occur, particularly in environments with heavy logging or compression activities.
The fix implemented addresses this by ensuring that EXTENT_FLAG_LOGGING is properly cleared from the local 'flags' copy used during split operations, while maintaining the existing behavior of only clearing EXTENT_FLAG_PINNED from em->flags. This restoration of pre-commit behavior ensures that split extent maps correctly inherit a clean flag state, preventing the propagation of logging flags that would otherwise persist until manual cleanup or system termination. The solution aligns with established kernel security practices by maintaining proper flag management and ensuring memory structures are properly cleaned up before deallocation. This vulnerability demonstrates the critical importance of careful flag handling in kernel memory management operations and highlights how seemingly minor changes to flag clearing mechanisms can introduce severe stability issues.
This vulnerability can be categorized under CWE-459 as "Incomplete Cleanup" and relates to improper resource management in kernel space. From an ATT&CK perspective, this represents a potential privilege escalation vector through kernel memory corruption (T1068) and system instability (T1499). The fix ensures proper flag propagation and cleanup mechanisms while maintaining the intended compression type identification functionality introduced in the problematic commit.