Re: [RFC PATCH] tracing: Fix syscall tracepoint use-after-free

[Mathieu Desnoyers]

On 2024-10-22 12:14, Jordan Rife wrote:
> I assume this patch isn't meant to fix the related issues with freeing
> BPF programs/links with call_rcu?

No, indeed. I notice that bpf_link_free() uses a prog->sleepable flag to
choose between:

                if (sleepable)
                        call_rcu_tasks_trace(&link->rcu, bpf_link_defer_dealloc_mult_rcu_gp);
                else
                        call_rcu(&link->rcu, bpf_link_defer_dealloc_rcu_gp);

But the faultable syscall tracepoint series does not require syscall programs
to be sleepable. So some changes may be needed on the ebpf side there.

> On the BPF side I think there needs to be some smarter handling of
> when to use call_rcu or call_rcu_tasks_trace to free links/programs
> based on whether or not the program type can be executed in this
> context. Right now call_rcu_tasks_trace is used if the program is
> sleepable, but that isn't necessarily the case here. Off the top of my
> head this would be BPF_PROG_TYPE_RAW_TRACEPOINT and
> BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE, but may extend to
> BPF_PROG_TYPE_TRACEPOINT? I'll let some of the BPF folks chime in
> here, as I'm not entirely sure.

A big hammer solution would be to make all grace periods waited for after
a bpf tracepoint probe unregister chain call_rcu and call_rcu_tasks_trace.

Else, if we properly tag all programs attached to syscall tracepoints as
sleepable, then keeping the call_rcu_tasks_trace() only for those would
work.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com