Vulnerability CVE-2024-26737
Published: 2024-04-03

Description:
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.

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