CVE-2024-26737

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

bpf: Fix racing between bpf_timer_cancel_and_free and bpf_timer_cancel

The following race is possible between bpf_timer_cancel_and_free and bpf_timer_cancel. It will lead a UAF on the timer->timer.

bpf_timer_cancel(); spin_lock(); t = timer->time; spin_unlock();

				bpf_timer_cancel_and_free();
					spin_lock();
					t = timer->timer;
					timer->timer = NULL;
					spin_unlock();
					hrtimer_cancel(&t->timer);
					kfree(t);

/* UAF on t */
hrtimer_cancel(&t->timer);

In bpf_timer_cancel_and_free, this patch frees the timer->timer after a rcu grace period. This requires a rcu_head addition to the struct bpf_hrtimer. Another kfree(t) happens in bpf_timer_init, this does not need a kfree_rcu because it is still under the spin_lock and timer->timer has not been visible by others yet.

In bpf_timer_cancel, rcu_read_lock() is added because this helper can be used in a non rcu critical section context (e.g. from a sleepable bpf prog). Other timer->timer usages in helpers.c have been audited, bpf_timer_cancel() is the only place where timer->timer is used outside of the spin_lock.

Another solution considered is to mark a t->flag in bpf_timer_cancel and clear it after hrtimer_cancel() is done. In bpf_timer_cancel_and_free, it busy waits for the flag to be cleared before kfree(t). This patch goes with a straight forward solution and frees timer->timer after a rcu grace period.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/0281b919e175bb9c3128bd3872ac2903e9436e3f
• https://git.kernel.org/stable/c/5268bb02107b9eedfdcd51db75b407d10043368c
• https://git.kernel.org/stable/c/7d80a9e745fa5b47da3bca001f186c02485c7c33
• https://git.kernel.org/stable/c/8327ed12e8ebc5436bfaa1786c49988894f9c8a6
• https://git.kernel.org/stable/c/addf5e297e6cbf5341f9c07720693ca9ba0057b5
• https://git.kernel.org/stable/c/0281b919e175bb9c3128bd3872ac2903e9436e3f
• https://git.kernel.org/stable/c/5268bb02107b9eedfdcd51db75b407d10043368c
• https://git.kernel.org/stable/c/7d80a9e745fa5b47da3bca001f186c02485c7c33
• https://git.kernel.org/stable/c/8327ed12e8ebc5436bfaa1786c49988894f9c8a6
• https://git.kernel.org/stable/c/addf5e297e6cbf5341f9c07720693ca9ba0057b5