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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-10-22 15:53, Andrii Nakryiko wrote:
> On Tue, Oct 22, 2024 at 10:55 AM Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> 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.
> 
> Your fix now adds a chain of call_rcu -> call_rcu_tasks_trace ->
> kfree, which should work regardless of sleepable/non-sleepable. For
> the BPF-side, yes, we do different things depending on prog->sleepable
> (adding extra call_rcu_tasks_trace for sleepable, while still keeping
> call_rcu in the chain), so the BPF side should be good, I think.
> 
>>
>>>
>>> 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.
>>
> 
>  From the BPF standpoint, as of right now, neither of RAW_TRACEPOINT or
> TRACEPOINT programs are sleepable. So a single RCU grace period is
> fine. But even if they were (and we'll allow that later on), we handle
> sleepable programs with the same call_rcu_tasks_trace -> call_rcu
> chain.

Good points, in this commit:

commit 4aadde89d8 ("tracing/bpf: disable preemption in syscall probe")
I took care to disable preemption around use of the bpf program attached
to a syscall tracepoint, which makes this change a no-op from the
tracers' perspective.

It's only when you'll decide to remove this preempt-off and allow
syscall tracepoints to sleep in bpf that you'll need to tweak that.

> 
> That's just to say that I don't think that we need any BPF-specific
> fix beyond what Mathieu is doing in this patch, so:
> 
> Acked-by: Andrii Nakryiko <andrii@kernel.org>

Thanks!

Mathieu

> 
> 
>> 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
>>

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