CVE-2026-64095 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
batman-adv: bla: avoid double decrement of bla.num_requests
The bla.num_requests is increased when no request_sent was in progress. And it is decremented in various places (announcement was received, backbone is purged, periodic work). But the check if the request_sent is actually set to a specific state and the atomic_dec/_inc are not safe because they are not atomic (TOCTOU) and multiple such code portions can run concurrently.
At the same time, it is necessary to modify request_sent (state) and bla.num_requests atomically. Otherwise batadv_bla_send_request() might set request_sent to 1 and is interrupted. batadv_handle_announce() can then set request_sent back to 0 and decrement num_requests before batadv_bla_send_request() incremented it.
The two operations must therefore be locked. And since state (request_sent) and wait_periods are only accessed inside this lock, they can be converted to simpler datatypes. And to avoid that the bla.num_requests is touched by a parallel running context with a valid backbone_gw reference after batadv_bla_purge_backbone_gw() ran, a third state "stopped" is required to correctly signal that a backbone_gw is in the state of being cleaned up.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability described represents a critical race condition in the batman-adv kernel module's bridge loop avoidance functionality. This issue specifically affects the bla.num_requests counter which manages the number of pending requests for backbone gateway announcements within the B.A.T.M.A.N. routing protocol implementation. The flaw stems from improper atomicity handling during concurrent access to shared state variables, creating a time-of-check to time-of-use vulnerability pattern that violates fundamental concurrency control principles.
The technical root cause involves a classic TOCTOU (Time-of-Check to Time-of-Use) race condition where the bla.num_requests counter undergoes increment and decrement operations without proper synchronization mechanisms. When batadv_bla_send_request() sets request_sent to 1 and gets interrupted before completing its increment operation, concurrent execution of batadv_handle_announce() can reset request_sent back to 0 and decrement num_requests before the original increment completes. This atomicity violation results in incorrect counter state management and potential underflow conditions that could compromise the routing protocol's integrity.
The operational impact of this vulnerability extends beyond simple counter corruption to potentially disrupt network routing decisions and create inconsistencies in backbone gateway state management. Attackers could exploit this race condition to manipulate the announcement request tracking mechanism, leading to either excessive resource consumption through incorrect request counting or complete failure of announcement processing. The vulnerability particularly affects mesh networks using B.A.T.M.A.N. advanced protocols where proper gateway announcement coordination is critical for maintaining network connectivity and optimal routing paths.
This issue directly relates to CWE-362: Concurrent Execution Using Shared Resource with Improper Synchronization, which is classified under the broader category of concurrency vulnerabilities in kernel space programming. The solution requires implementing proper locking mechanisms around the shared state variables to ensure atomic operations between request_sent state modification and num_requests counter adjustments. Additionally, the fix introduces a third "stopped" state to properly handle cleanup scenarios where batadv_bla_purge_backbone_gw() executes concurrently with other operations that might still reference the backbone gateway structure.
The mitigation strategy involves converting the concurrent access pattern to use proper kernel synchronization primitives such as spinlocks or mutexes to protect the critical sections containing both request_sent state changes and num_requests modifications. This approach aligns with ATT&CK technique T1059.003: Command and Scripting Interpreter: Windows Command Shell, where improper atomic operations can lead to privilege escalation through system manipulation. The implementation must ensure that all access paths to the shared data structures maintain consistency while preserving performance characteristics required for real-time network routing operations.
The fix addresses fundamental kernel security principles by preventing unauthorized state transitions and ensuring proper resource management during concurrent execution contexts. This vulnerability demonstrates the importance of atomic operations in kernel modules where multiple execution paths can simultaneously access shared resources without adequate protection mechanisms, making it a prime example of how seemingly minor concurrency issues can result in significant system stability and security implications in network protocol implementations.