CVE-2026-98644 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Avoid soft lockup in __htab_map_lookup_and_delete_batch()
__htab_map_lookup_and_delete_batch() has no rescheduling point. The batch count bounds how many entries are copied out, not how many buckets are visited, so one BPF_MAP_LOOKUP_BATCH call can walk the map end to end. The empty-bucket fast path is worse: it stays inside a single rcu_read_lock() / bpf_disable_instrumentation() section for any run of consecutive empty buckets.
That holds up on small maps, but it falls apart at scale. On a 144-CPU arm64 host running a CONFIG_PREEMPT_NONE kernel, periodic BPF_MAP_LOOKUP_BATCH calls against an LRU hash map with 16,777,216 buckets held a CPU inside the batch op for 77+ seconds and triggered the soft lockup watchdog.
Commit 75134f16e7dd ("bpf: Add schedule points in batch ops") fixed this same problem in the generic batch ops, but not in this htab-native path, which every htab-based hash map variant uses for its lookup[_and_delete]
batch ops.
Complete that fix here. Leave the critical section after 64 consecutive empty buckets, call cond_resched_tasks_rcu_qs(), and resume at the saved bucket cursor. No locks are held at that point, and resuming from the cursor is already the function's behavior for non-empty buckets. Add the same call to the per-bucket loop after copy_to_user(), where every lock has been dropped. cond_resched_rcu() is not enough here: sleeping with bpf_prog_active elevated makes tracing programs on that CPU silently skip their invocations.
Plain cond_resched() is not enough either. It is a no-op under PREEMPT and PREEMPT_LAZY, the only models arm64 and x86 have offered since commit 7dadeaa6e851 ("sched: Further restrict the preemption modes"). It is also never a Tasks RCU quiescent state, in any model: the reschedule counts as a preemption. The walking task stays a holdout and stalls every synchronize_rcu_tasks() caller, ftrace and BPF trampoline teardown included, until the syscall returns [1].
cond_resched_tasks_rcu_qs() is the usual tool for that [2]. It reports
the quiescent state at each yield and still reschedules as cond_resched() does on PREEMPT_NONE and PREEMPT_VOLUNTARY kernels.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel contains a vulnerability within the BPF hash table implementation, specifically in the __htab_map_lookup_and_delete_batch function, which can lead to soft lockups under specific conditions. This issue arises because the batch operation lacks necessary rescheduling points during its execution loop. The design of this function bounds the number of entries copied out rather than limiting the number of buckets visited, allowing a single BPF_MAP_LOOKUP_BATCH call to traverse an entire map if required. While small maps may not exhibit severe symptoms due to their limited size, large-scale deployments expose critical flaws in the kernel's scheduling and preemption handling mechanisms.
The technical flaw is most pronounced when encountering consecutive empty buckets within the hash table structure. In such scenarios, the code remains inside a single rcu_read_lock and bpf_disable_instrumentation section for extended periods without yielding control to other tasks or checking for pending work. This behavior prevents the kernel from performing necessary housekeeping tasks, including updating Tasks RCU quiescent states. On systems with high CPU counts, such as 144-CPU arm64 hosts running CONFIG_PREEMPT_NONE kernels, periodic batch lookups against large LRU hash maps can hold a CPU for over seventy seconds. This duration exceeds the threshold of the soft lockup watchdog, triggering system alerts and potentially causing broader instability or denial of service conditions where other processes are starved of CPU time.
The root cause lies in the omission of proper scheduling calls within critical sections that disable preemption and instrumentation. Previous fixes addressed similar issues in generic batch operations but failed to apply them to this htab-native path, which is utilized by all hash map variants for lookup and delete batches. The absence of rescheduling points means that even when no data is being processed due to empty buckets, the CPU remains locked in a non-preemptible state. This prevents other tasks from running and blocks RCU callbacks from completing, effectively stalling synchronize_rcu_tasks callers such as ftrace and BPF trampoline teardown processes until the syscall finally returns.
Mitigation of this vulnerability requires inserting appropriate rescheduling calls at strategic points within the batch operation loop. Specifically, after processing sixty-four consecutive empty buckets or following each successful copy_to_user operation where locks have been dropped, the kernel must invoke cond_resched_tasks_rcu_qs(). This function is critical because it not only allows for preemption and scheduling but also reports a Tasks RCU quiescent state at each yield point. Standard alternatives like plain cond_resched are insufficient as they act as no-ops under modern PREEMPT and PREEMPT_LAZY models used by arm64 and x86 architectures, and they do not satisfy the requirements for advancing RCU tasks grace periods.
This vulnerability aligns with CWE-770: Allocation of Resources Without Limits or Throttling, as the operation can consume unlimited CPU time without yielding to other system activities. From an ATT&CK perspective, this relates to T1496: Resource Hijacking, where a process consumes excessive resources leading to denial of service for legitimate users. The fix ensures that long-running batch operations remain responsive and compliant with kernel scheduling policies, maintaining system stability even under heavy load or large data structures. Developers should ensure their BPF programs utilize updated kernels containing this patch to prevent potential soft lockups in production environments relying on hash map batching features.