CVE-2026-89991
UnknownSummary
A bug was fixed in the Linux kernel's BPF pcpu_freelist mechanism that caused an infinite loop in __pcpu_freelist_push(). The issue occurs when only one CPU is possible (e.g. CONFIG_SMP=n, nr_cpus=1 or possible_cpus=1) and an NMI re-enters pcpu_freelist_push() while the interrupted context holds that CPU's freelist lock. The fallback loop then has no other possible CPU to examine and can never acquire the lock.
Risk Assessment
On systems with a single possible CPU (including uniprocessor configurations), the kernel may hang in an infinite loop, leading to a denial of service. The risk mainly affects environments using BPF programs and NMI/perf events.
Recommendation
Update the Linux kernel to a version containing the fix that restores the extra fallback freelist head and checks it in the pop path. If updating is not possible, consider avoiding single-possible-CPU configurations combined with heavy BPF usage.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: bpf: Fix infinite loop in pcpu_freelist push with one possible CPU __pcpu_freelist_push() can loop forever when only one CPU is possible and an NMI re-enters pcpu_freelist_push() while the interrupted context holds that CPU's freelist lock. After the current-CPU fast path fails, the fallback loop walks cpu_possible_mask while skipping the current CPU. With CONFIG_SMP=n, or when an SMP kernel is limited to one possible CPU with nr_cpus=1 or possible_cpus=1, there are no other possible CPUs to examine. The loop therefore makes no lock acquisition attempt and can never make progress. The following stack was observed on a UP system: NMI context: pcpu_freelist_push free_htab_elem htab_map_delete_elem [perf-event BPF program] __perf_event_overflow perf_event_nmi_handler exc_nmi Interrupted context: __pcpu_freelist_push pcpu_freelist_push free_htab_elem htab_map_delete_elem [raw_tp/sys_enter BPF program] __bpf_trace_sys_enter do_syscall_64 raw_res_spin_lock() detects the same-CPU recursive acquisition and returns -EDEADLK, but the subsequent fallback loop has no candidate head on a system with one possible CPU. Restore the extra fallback head that existed before the rqspinlock conversion. Keep the current-CPU fast path, then try the other possible CPUs and finally the extra head. The additional head lets a push, which cannot fail without losing a preallocated element, make progress when the only per-CPU head is held by the interrupted context. Also check the extra head from the pop path so that nodes placed there can be reused.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS