Executive Summary

CVE-2024-41045 exposes a deadlock and use‑after‑free flaw in the Linux kernel’s BPF timer subsystem. When a timer is cancelled via bpf_map_update_elem or async_cancel_and_free, concurrent callbacks can re‑arm the timer before it is freed, causing the RCU grace period to release the hrtimer while it is still queued. This race can lead to kernel crashes or denial‑of‑service. The patch defers cancellation to a global workqueue, preventing deadlock and eliminating the UAF.

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Authoritative CVE Metadata - CVSS Base Score: 7.8 (HIGH) - Published: 2024-07-29T15:15:12.873 - Last Modified: 2026-10-03T11:17:30.193

Original Description: In the Linux kernel, the following vulnerability has been resolved:

bpf: Defer work in bpf_timer_cancel_and_free

Currently, the same case as previous patch (two timer callbacks trying to cancel each other) can be invoked through bpf_map_update_elem as well, or more precisely, freeing map elements containing timers. Since this relies on hrtimer_cancel as well, it is prone to the same deadlock situation as the previous patch.

It would be sufficient to use hrtimer_try_to_cancel to fix this problem, as the timer cannot be enqueued after async_cancel_and_free. Once async_cancel_and_free has been done, the timer must be reinitialized before it can be armed again. The callback running in parallel trying to arm the timer will fail, and freeing bpf_hrtimer without waiting is sufficient (given kfree_rcu), and bpf_timer_cb will return HRTIMER_NORESTART, preventing the timer from being rearmed again.

However, there exists a UAF scenario where the callback arms the timer before entering this function, such that if cancellation fails (due to timer callback invoking this routine, or the target timer callback running concurrently). In such a case, if the timer expiration is significantly far in the future, the RCU grace period expiration happening before it will free the bpf_hrtimer state and along with it the struct hrtimer, that is enqueued.

Hence, it is clear cancellation needs to occur after async_cancel_and_free, and yet it cannot be done inline due to deadlock issues. We thus modify bpf_timer_cancel_and_free to defer work to the global workqueue, adding a work_struct alongside rcu_head (both used at different points of time, so can share space).

Update existing code comments to reflect the new state of affairs.

"It's easier to see the mistakes on someone else's paper."

— Unknown