CVE-2022-49999 in Linux
Summary
by MITRE • 06/18/2025
In the Linux kernel, the following vulnerability has been resolved:
btrfs: fix space cache corruption and potential double allocations
When testing space_cache v2 on a large set of machines, we encountered a few symptoms:
1. "unable to add free space :-17" (EEXIST) errors. 2. Missing free space info items, sometimes caught with a "missing free space info for X" error. 3. Double-accounted space: ranges that were allocated in the extent tree and also marked as free in the free space tree, ranges that were marked as allocated twice in the extent tree, or ranges that were marked as free twice in the free space tree. If the latter made it onto disk, the next reboot would hit the BUG_ON() in add_new_free_space(). 4. On some hosts with no on-disk corruption or error messages, the in-memory space cache (dumped with drgn) disagreed with the free space tree.
All of these symptoms have the same underlying cause: a race between caching the free space for a block group and returning free space to the in-memory space cache for pinned extents causes us to double-add a free range to the space cache. This race exists when free space is cached from the free space tree (space_cache=v2) or the extent tree (nospace_cache, or space_cache=v1 if the cache needs to be regenerated). struct btrfs_block_group::last_byte_to_unpin and struct btrfs_block_group::progress are supposed to protect against this race, but commit d0c2f4fa555e ("btrfs: make concurrent fsyncs wait less when waiting for a transaction commit") subtly broke this by allowing multiple transactions to be unpinning extents at the same time.
Specifically, the race is as follows:
1. An extent is deleted from an uncached block group in transaction A. 2. btrfs_commit_transaction() is called for transaction A. 3. btrfs_run_delayed_refs() -> __btrfs_free_extent() runs the delayed ref for the deleted extent. 4. __btrfs_free_extent() -> do_free_extent_accounting() -> add_to_free_space_tree() adds the deleted extent back to the free space tree. 5. do_free_extent_accounting() -> btrfs_update_block_group() -> btrfs_cache_block_group() queues up the block group to get cached. block_group->progress is set to block_group->start. 6. btrfs_commit_transaction() for transaction A calls switch_commit_roots(). It sets block_group->last_byte_to_unpin to block_group->progress, which is block_group->start because the block group hasn't been cached yet. 7. The caching thread gets to our block group. Since the commit roots were already switched, load_free_space_tree() sees the deleted extent as free and adds it to the space cache. It finishes caching and sets block_group->progress to U64_MAX. 8. btrfs_commit_transaction() advances transaction A to TRANS_STATE_SUPER_COMMITTED. 9. fsync calls btrfs_commit_transaction() for transaction B. Since transaction A is already in TRANS_STATE_SUPER_COMMITTED and the commit is for fsync, it advances. 10. btrfs_commit_transaction() for transaction B calls switch_commit_roots(). This time, the block group has already been cached, so it sets block_group->last_byte_to_unpin to U64_MAX. 11. btrfs_commit_transaction() for transaction A calls btrfs_finish_extent_commit(), which calls unpin_extent_range() for the deleted extent. It sees last_byte_to_unpin set to U64_MAX (by transaction B!), so it adds the deleted extent to the space cache again!
This explains all of our symptoms above:
* If the sequence of events is exactly as described above, when the free space is re-added in step 11, it will fail with EEXIST. * If another thread reallocates the deleted extent in between steps 7 and 11, then step 11 will silently re-add that space to the space cache as free even though it is actually allocated. Then, if that space is allocated *again*, the free space tree will be corrupted (namely, the wrong item will be deleted). * If we don't catch this free space tree corr ---truncated---
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Analysis
by VulDB Data Team • 11/30/2025
The vulnerability described in CVE-2022-49999 affects the Linux kernel's Btrfs file system implementation, specifically within the space cache management subsystem. This issue manifests as a race condition that can lead to critical data corruption and inconsistent state management between the extent tree and free space tree components. The flaw occurs when handling space cache v2 operations on large-scale systems, where concurrent operations can cause overlapping memory management actions that result in double allocations and corrupted free space accounting. The vulnerability directly impacts the reliability and integrity of Btrfs storage systems, potentially leading to data loss, system instability, and incorrect space reporting.
The root cause of this vulnerability lies in the improper synchronization between multiple concurrent transactions attempting to unpin extents within the same block group. The race condition stems from a subtle regression introduced in commit d0c2f4fa555e which altered how concurrent fsync operations interact with transaction commit mechanisms. This regression allows multiple transactions to simultaneously unpin extents, creating a scenario where the last_byte_to_unpin and progress fields of block group structures become inconsistent. The specific sequence involves transaction A committing while transaction B is in progress, causing the unpinning logic to incorrectly interpret the block group's state and add the same free space range twice to the in-memory cache. This condition is particularly dangerous because it can occur without immediate error reporting, making detection difficult and potentially leading to cascading failures during subsequent system operations.
The operational impact of this vulnerability extends beyond simple data corruption to encompass potential system crashes and inconsistent storage state. When the race condition occurs, it can produce EEXIST errors indicating that free space cannot be added because it already exists, or more subtly, cause missing free space information items that are detected during system operations. The double-accounted space scenario creates a particularly insidious problem where ranges appear both allocated in the extent tree and free in the free space tree, leading to BUG_ON() failures during reboot operations. Additionally, the discrepancy between in-memory space cache and on-disk free space tree states can cause drgn debugging tools to report inconsistent data, making system diagnosis and recovery more challenging. This vulnerability affects systems using Btrfs with space_cache=v2 or nospace_cache configurations, and impacts both single and multi-threaded operations involving concurrent file system modifications.
Mitigation strategies for this vulnerability require immediate kernel updates to address the race condition in the Btrfs implementation. System administrators should prioritize applying the patched kernel version that resolves the concurrent transaction handling issue and restores proper synchronization between the block group state management and space cache operations. Additional protective measures include monitoring for error messages related to free space management and implementing regular file system checks to detect early signs of corruption. Organizations should also consider implementing more robust backup strategies and ensuring that critical Btrfs systems are regularly tested for consistency after applying patches. The vulnerability aligns with CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) and can be mapped to ATT&CK technique T1485 (Data Destruction) when the corruption leads to system instability or data loss. Given the severity of potential data corruption, organizations should perform thorough testing of patched systems before deployment to ensure that no regressions or additional issues have been introduced by the fix.