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

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Message-ID: <3362d414-4d6f-43a7-80af-1c72c5e66d70@efficios.com>
Date: Tue, 22 Oct 2024 13:54:01 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Jordan Rife <jrife@...gle.com>
Cc: Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
 syzbot+b390c8062d8387b6272a@...kaller.appspotmail.com,
 Michael Jeanson <mjeanson@...icios.com>,
 Masami Hiramatsu <mhiramat@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Alexei Starovoitov <ast@...nel.org>,
 Yonghong Song <yhs@...com>, "Paul E . McKenney" <paulmck@...nel.org>,
 Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Namhyung Kim <namhyung@...nel.org>,
 Andrii Nakryiko <andrii.nakryiko@...il.com>, bpf@...r.kernel.org,
 Joel Fernandes <joel@...lfernandes.org>, Alexei Starovoitov <ast@...nel.org>
Subject: Re: [RFC PATCH] tracing: Fix syscall tracepoint use-after-free

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