[PATCH bpf v5 1/3] bpf: Wait for an RCU tasks grace period before freeing trampoline progs

[bot+bpf-ci at kernel.org]

> bpf: Wait for an RCU tasks grace period before freeing trampoline progs
>
> When a prog is detached from a trampoline, it is freed after an RCU
> grace period, or an RCU tasks trace one if it is sleepable. This covers
> the tasks that are running the prog, since the prog's enter helper takes
> the matching read lock before the prog is called. On a preemptible
> kernel, it doesn't cover a task that was preempted in the trampoline
> just before the enter helper. That task holds no lock yet, and it calls
> the prog after it was freed:
>
>   BUG: KASAN: vmalloc-out-of-bounds in __bpf_prog_enter_recur+0x3a5/0x3f0
>   Read of size 8 at addr ffffc90000055040 by task candidate/110
>   CPU: 1 UID: 0 PID: 110 Comm: candidate Not tainted 7.3.0-rc2-00014-g15071f2a1263-dirty #2 PREEMPT(full)
>   Call Trace:
>    <TASK>
>    __bpf_prog_enter_recur+0x3a5/0x3f0
>    bpf_trampoline_6442509193+0x37/0xf1
>    __x64_sys_futex+0x9/0x410
>    do_syscall_64+0xb0/0x530
>    ...
>
> Wait for an RCU tasks grace period before the existing one when freeing
> a prog that was linked to a trampoline. An RCU tasks grace period only
> ends once the tasks that were preempted have run again, and
> bpf_tramp_image_put() already relies on it to free the image. It doesn't
> wait for tasks that sleep in the trampoline, the next commit takes care
> of those.
>
> Only progs that were linked to a trampoline can be called this way, so
> bpf_trampoline_add_prog() marks them and other progs are still freed as
> before.
>
> Fixes: e21aa341785c ("bpf: Fix fexit trampoline.")

Is this the right Fixes: target?

The race being closed here (a task preempted inside the trampoline image
before __bpf_prog_enter*() takes its RCU read lock, while the prog is freed
after only a regular RCU or RCU tasks trace grace period) appears to predate
e21aa341785c.

In fec56f5890d9 ("bpf: Introduce BPF trampoline") bpf_trampoline_update()
switched images with no tasks RCU wait, and __bpf_prog_put_noref() freed the
prog with a plain call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu).

Just before e21aa341785c, the synchronize_rcu_mult(call_rcu_tasks,
call_rcu_tasks_trace) in bpf_trampoline_update() ran before
arch_prepare_bpf_trampoline() rather than after the new image was installed,
and the total == 0 unregister_fentry() path had no wait at all, so a task
preempted in the old image could already reach a freed prog.

Would Fixes: fec56f5890d9 ("bpf: Introduce BPF trampoline") be more
accurate, so that stable trees without e21aa341785c also pick this up? The
follow-up 49ca60187380 in this series uses the same tag.


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