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

["Florent Revest" <florent.revest@linux.dev>]

On Sun Sep 27, 2026 at 8:41 PM UTC,  wrote:
> > 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.

Yeah, that seems right. It shouldn't make much of a difference for stable
because both have been introduced in the same 5.10.y. I can either switch this
if there's a v6 or a maintainer can change when applying.