CVE-2024-26737

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CVE-2024-26737

Summary: bpf: Fix racing between bpf_timer_cancel_and_free and bpf_timer_cancel

Details: 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/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://github.com/CVEProject/cvelistV5/tree/main/cves/2024/26xxx/CVE-2024-26737.json, https://nvd.nist.gov/vuln/detail/CVE-2024-26737, https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git