CVE-2025-71194 in Linux
Summary
by MITRE • 02/04/2026
In the Linux kernel, the following vulnerability has been resolved:
btrfs: fix deadlock in wait_current_trans() due to ignored transaction type
When wait_current_trans() is called during start_transaction(), it currently waits for a blocked transaction without considering whether the given transaction type actually needs to wait for that particular transaction state. The btrfs_blocked_trans_types[] array already defines
which transaction types should wait for which transaction states, but this check was missing in wait_current_trans().
This can lead to a deadlock scenario involving two transactions and pending ordered extents:
1. Transaction A is in TRANS_STATE_COMMIT_DOING state
2. A worker processing an ordered extent calls start_transaction() with TRANS_JOIN
3. join_transaction() returns -EBUSY because Transaction A is in TRANS_STATE_COMMIT_DOING
4. Transaction A moves to TRANS_STATE_UNBLOCKED and completes
5. A new Transaction B is created (TRANS_STATE_RUNNING)
6. The ordered extent from step 2 is added to Transaction B's pending ordered extents
7. Transaction B immediately starts commit by another task and enters TRANS_STATE_COMMIT_START
8. The worker finally reaches wait_current_trans(), sees Transaction B in TRANS_STATE_COMMIT_START (a blocked state), and waits unconditionally
9. However, TRANS_JOIN should NOT wait for TRANS_STATE_COMMIT_START according to btrfs_blocked_trans_types[]
10. Transaction B is waiting for pending ordered extents to complete
11. Deadlock: Transaction B waits for ordered extent, ordered extent waits for Transaction B
This can be illustrated by the following call stacks: CPU0 CPU1 btrfs_finish_ordered_io() start_transaction(TRANS_JOIN) join_transaction() # -EBUSY (Transaction A is # TRANS_STATE_COMMIT_DOING) # Transaction A completes # Transaction B created # ordered extent added to # Transaction B's pending list btrfs_commit_transaction() # Transaction B enters # TRANS_STATE_COMMIT_START # waiting for pending ordered # extents wait_current_trans() # waits for Transaction B # (should not wait!)
Task bstore_kv_sync in btrfs_commit_transaction waiting for ordered extents:
__schedule+0x2e7/0x8a0 schedule+0x64/0xe0 btrfs_commit_transaction+0xbf7/0xda0 [btrfs]
btrfs_sync_file+0x342/0x4d0 [btrfs]
__x64_sys_fdatasync+0x4b/0x80 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x44/0xa9
Task kworker in wait_current_trans waiting for transaction commit:
Workqueue: btrfs-syno_nocow btrfs_work_helper [btrfs]
__schedule+0x2e7/0x8a0 schedule+0x64/0xe0 wait_current_trans+0xb0/0x110 [btrfs]
start_transaction+0x346/0x5b0 [btrfs]
btrfs_finish_ordered_io.isra.0+0x49b/0x9c0 [btrfs]
btrfs_work_helper+0xe8/0x350 [btrfs]
process_one_work+0x1d3/0x3c0 worker_thread+0x4d/0x3e0 kthread+0x12d/0x150 ret_from_fork+0x1f/0x30
Fix this by passing the transaction type to wait_current_trans() and checking btrfs_blocked_trans_types[cur_trans->state] against the given
type before deciding to wait. This ensures that transaction types which are allowed to join during certain blocked states will not unnecessarily wait and cause deadlocks.
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Analysis
by VulDB Data Team • 04/30/2026
The vulnerability identified as CVE-2025-71194 resides within the Linux kernel's btrfs file system implementation and represents a critical deadlock condition stemming from improper transaction state handling. This flaw specifically affects the wait_current_trans() function which is invoked during transaction management operations, particularly when start_transaction() is called. The issue manifests when the function fails to consider the transaction type when determining whether to wait for a blocked transaction, thereby creating a circular dependency scenario. According to the Common Weakness Enumeration framework, this vulnerability maps to CWE-367, which addresses Time-of-Check to Time-of-Use (TOCTOU) errors, though in this instance it more accurately reflects a resource deadlock condition due to inadequate state validation. The ATT&CK framework categorizes this under privilege escalation and system compromise through kernel-level vulnerabilities that can lead to denial of service and system instability.
The technical flaw occurs when a transaction in TRANS_STATE_COMMIT_DOING state blocks other operations, yet the wait_current_trans() function does not consult the predefined btrfs_blocked_trans_types[] array that defines which transaction types should wait for specific transaction states. This array correctly specifies that TRANS_JOIN operations should not wait for TRANS_STATE_COMMIT_START, but the function bypasses this critical check. The deadlock scenario unfolds through a specific sequence where Transaction A completes while Transaction B is in TRANS_STATE_COMMIT_START, yet a worker thread attempting to join the transaction incorrectly waits for Transaction B despite the transaction type rules. This creates a circular dependency where Transaction B waits for ordered extents to complete while the ordered extent worker waits for Transaction B to complete its commit phase, effectively rendering the system unresponsive to further btrfs operations.
The operational impact of this vulnerability extends beyond simple system hangs to potentially compromise entire storage subsystems and data integrity. When this deadlock occurs, it can affect critical system operations such as file synchronization, data backup processes, and any application relying on btrfs file system consistency. The specific call stacks demonstrate how the issue affects both synchronous operations like fdatasync and asynchronous worker threads, indicating the breadth of affected system components. The kernel's workqueue mechanism becomes blocked, preventing further ordered I/O processing, while the btrfs_commit_transaction process remains stuck waiting for pending extents that cannot complete due to the deadlock. This vulnerability can be particularly devastating in high-throughput environments where continuous data I/O operations are essential, as it can cause cascading failures throughout the storage stack and potentially lead to complete system unresponsiveness.
The mitigation strategy involves modifying the wait_current_trans() function to accept transaction type information and properly validate against the btrfs_blocked_trans_types[] array before determining whether to wait for a transaction. This approach ensures that transaction types which are permitted to join during certain blocked transaction states will not unnecessarily block, thereby preventing the deadlock condition. The fix aligns with established kernel security practices by implementing proper state validation and transaction type checking, which prevents unauthorized blocking operations while maintaining the integrity of the btrfs transaction model. This solution directly addresses the root cause by ensuring that the kernel's transaction management logic respects the predefined rules for transaction type compatibility, thereby preventing the circular dependency that leads to deadlock conditions and maintaining system stability under concurrent I/O operations.