CVE-2024-42239 bpf: Fail bpf_timer_cancel when callback is being cancelled
A race condition in the Linux kernel BPF timer cancellation logic can cause a deadlock and system lockup when multiple timers cancel each other.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2024-42239?
CVE-2024-42239 is a medium-severity vulnerability in Linux kernel, published on 7 Aug 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
bpf: Fail bpf_timer_cancel when callback is being cancelled
Given a schedule:
timer1 cb timer2 cb
bpf_timer_cancel(timer2); bpf_timer_cancel(timer1);
Both bpf_timer_cancel calls would wait for the other callback to finish executing, introducing a lockup.
Add an atomic_t count named 'cancelling' in bpf_hrtimer. This keeps track of all in-flight cancellation requests for a given BPF timer. Whenever cancelling a BPF timer, we must check if we have outstanding cancellation requests, and if so, we must fail the operation with an error (-EDEADLK) since cancellation is synchronous and waits for the callback to finish executing. This implies that we can enter a deadlock situation involving two or more timer callbacks executing in parallel and attempting to cancel one another.
Note that we avoid incrementing the cancelling counter for the target timer (the one being cancelled) if bpf_timer_cancel is not invoked from a callback, to avoid spurious errors. The whole point of detecting cur->cancelling and returning -EDEADLK is to not enter a busy wait loop (which may or may not lead to a lockup). This does not apply in case the caller is in a non-callback context, the other side can continue to cancel as it sees fit without running into errors.
Background on prior attempts:
Earlier versions of this patch used a bool 'cancelling' bit and used the following pattern under timer->lock to publish cancellation status.
lock(t->lock); t->cancelling = true; mb(); if (cur->cancelling) return -EDEADLK; unlock(t->lock); hrtimer_cancel(t->timer); t->cancelling = false;
The store outside the critical section could overwrite a parallel requests t->cancelling assignment to true, to ensure the parallely executing callback observes its cancellation status.
It would be necessary to clear this cancelling bit once hrtimer_cancel is done, but lack of serialization introduced races. Another option was explored where bpf_timer_start would clear the bit when (re)starting the timer under timer->lock. This would ensure serialized access to the cancelling bit, but may allow it to be cleared before in-flight hrtimer_cancel has finished executing, such that lockups can occur again.
Thus, we choose an atomic counter to keep track of all outstanding cancellation requests and use it to prevent lockups in case callbacks attempt to cancel each other while executing in parallel.
Weakness: CWE-667
Which versions are affected by CVE-2024-42239?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2024-42239?
Apply the vendor's patch.
Is CVE-2024-42239 being exploited?
There is no known exploitation of CVE-2024-42239 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-47453, AZL-47604, BELL-CVE-2024-42239, DEBIAN-CVE-2024-42239, GHSA-ggh4-5hvc-554r, ROOT-OS-DEBIAN-12-CVE-2024-42239 and 2 more.