Race Condition Affecting kernel-rt package, versions *

Note: Versions mentioned in the description apply only to the upstream kernel-rt package and not the kernel-rt package as distributed by RHEL. See How to fix? for RHEL:9 relevant fixed versions and status.

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.

• https://access.redhat.com/security/cve/CVE-2024-26737
• 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