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

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

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