CVE-2026-98398 in Linux
Summary
by MITRE • 10/14/2026
In the Linux kernel, the following vulnerability has been resolved:
locking/lockdep: Invalidate stale class_cache entries for zapped classes
syzbot reported a lockdep splat hitting DEBUG_LOCKS_WARN_ON(1) in hlock_class() due to an invalid class_idx:
WARNING: kernel/locking/lockdep.c:238 at __lock_acquire+0x382/0x2cf0 kernel/locking/lockdep.c:5203 Workqueue: wg-crypt-wg0 wg_packet_tx_worker RIP: 0010:hlock_class kernel/locking/lockdep.c:238 [inline]
RIP: 0010:check_wait_context kernel/locking/lockdep.c:4870 [inline]
RIP: 0010:__lock_acquire+0x389/0x2cf0 kernel/locking/lockdep.c:5203 Call Trace: <IRQ> lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5886 _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:173 tcp_tsq_handler+0x29/0x200 net/ipv4/tcp_output.c:1291 tcp_tsq_workfn+0x384/0x410 net/ipv4/tcp_output.c:1325 ...
When a lock class is zapped (e.g. during module unload or key unregistration), zap_class() clears the class's bit in lock_classes_in_use and removes it from the class hash table. However, existing lockdep_map instances embedded in data structures may still retain a pointer to the zapped class in their class_cache[]
array.
When __lock_acquire() subsequently runs on such a lock, it finds lock->class_cache[subclass] != NULL, skipping register_lock_class()
and assigning hlock->class_idx to the index of the zapped class. When check_wait_context() or hlock_class() inspects the held_lock, it finds !test_bit(class_idx, lock_classes_in_use) and warns. Furthermore, if the zapped slot is subsequently re-allocated to an unrelated lock key, the stale class_cache entry would erroneously match the unrelated class (ABA issue).
Add lock_class_cache_is_valid() to validate that the cached class is within lock_classes bounds, still allocated in lock_classes_in_use (using uninstrumented arch_test_bit() in __always_inline context so it is safe in noinstr contexts like match_held_lock()), and that class->key matches the expected subkey (taking lockdep_set_subclass() overrides into account). Also use READ_ONCE()/WRITE_ONCE() when accessing class_cache[]. If the entry is invalid or stale, fall back
to register_lock_class() / look_up_lock_class().
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Analysis
by VulDB Data Team • 10/15/2026
The Linux kernel's locking dependency validator, known as lockdep, relies on maintaining accurate state information about locks to detect potential deadlocks and other concurrency issues. A critical flaw was identified in how lockdep handles the lifecycle of lock classes, specifically during their removal or zapping phase. When a lock class is removed from the system, typically due to module unloading or key unregistration, the function zap_class() correctly clears the corresponding bit in the lock_classes_in_use bitmap and removes the entry from the class hash table. However, this cleanup process failed to invalidate existing references held by other data structures that embed lockdep_map instances. These embedded maps maintain a cache of previously seen classes within their class_cache array to optimize subsequent locking operations. Consequently, after a class is zapped, these cached pointers remain valid in memory but point to deallocated or invalid kernel objects, creating a stale reference scenario.
This architectural oversight leads to severe operational instability when the __lock_acquire function attempts to process locks associated with these stale entries. Because the cache still indicates that a class exists for a given subclass index, lockdep skips the necessary registration and lookup procedures. Instead, it directly assigns an invalid or outdated class index to the held_lock structure. Subsequent validation functions such as check_wait_context() and hlock_class() then inspect this held_lock and detect that the referenced class index is no longer marked as in use within the global bitmap. This discrepancy triggers a fatal kernel warning via DEBUG_LOCKS_WARN_ON, resulting in a lockdep splat that can crash the system or severely disrupt normal operation. The issue was initially reported by syzbot through automated fuzzing, highlighting its potential to be triggered under specific concurrency conditions involving network workqueues and TCP subsystems.
Beyond immediate crashes, this vulnerability introduces a serious security risk classified as an ABA problem within the lock dependency tracking mechanism. If the memory slot associated with the zapped class is subsequently reallocated by the kernel allocator for use by an unrelated lock key, the stale pointer in the class_cache array will erroneously match this new object. This misalignment causes lockdep to associate locking behavior and dependencies with incorrect locks, potentially masking genuine deadlocks or generating false positives that degrade system reliability. The lack of proper validation allows attackers or buggy drivers to exploit these inconsistencies, leading to undefined behavior in critical kernel paths such as TCP transmission queues where spinlock acquisition is frequent and timing-sensitive.
To mitigate this vulnerability, the Linux kernel developers implemented a robust validation mechanism via the new lock_class_cache_is_valid function. This helper performs rigorous checks before accepting cached class references. It verifies that the cached class index falls within valid bounds for lock_classes and confirms that the corresponding bit remains set in the lock_classes_in_use bitmap using uninstrumented arch_test_bit to ensure safety in noinstr contexts like match_held_lock(). Additionally, it validates that the key associated with the cached class matches the expected subkey, accounting for any subclass overrides applied via lockdep_set_subclass. The implementation also employs READ_ONCE and WRITE_ONCE macros when accessing the class_cache array to prevent compiler optimizations from causing race conditions or torn reads during concurrent access. If any of these validation checks fail, indicating a stale or invalid entry, the code gracefully falls back to registering the lock class anew through register_lock_class and look_up_lock_class, ensuring that only current and valid state information is used for dependency tracking.
From a standards perspective, this vulnerability aligns with CWE-416 Use After Free, as it involves accessing memory associated with an object after its lifetime has ended or been invalidated by the system. The resulting behavior also touches upon CWE-823 Object Reference Without Valid Usage, where the lockdep subsystem operates on references that do not correspond to active resources. In terms of attack vectors and detection techniques relevant to MITRE ATT&CK for Enterprise, this flaw relates to T1059 Command and Scripting Interpreter if exploited via kernel modules or scripts triggering module loads/unloads rapidly, though more accurately it represents a logic error in resource management akin to improper cleanup procedures often seen in advanced persistent threats targeting system stability. The fix ensures that the integrity of the locking dependency graph is maintained even under high churn conditions involving dynamic module loading and unloading, thereby preserving the effectiveness of lockdep as a debugging tool without introducing new failure modes into the kernel's concurrency control infrastructure.