CVE-2026-89991

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 CPUs 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.

Affected Software

References

• https://git.kernel.org/stable/c/a24a146ae2cb5d9faeea96d10010fcb6579abe1f
• https://git.kernel.org/stable/c/a5b5e62baceff9e252c6535cf42a027d04f226cf
• https://git.kernel.org/stable/c/efebf6496685c93150df5bb0794363ae70c5f58a