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From: Peter Zijlstra <peterz@infradead.org>
To: Jann Horn <jannh@google.com>
Cc: bpf@vger.kernel.org, live-patching@vger.kernel.org,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Steven Rostedt <rostedt@goodmis.org>,
	KP Singh <kpsingh@chromium.org>,
	Andy Lutomirski <luto@amacapital.net>,
	kernel list <linux-kernel@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Ingo Molnar <mingo@redhat.com>
Subject: Re: BPF tracing trampoline synchronization between update/freeing and execution?
Date: Mon, 6 Jan 2020 17:56:54 +0100
Message-ID: <20200106165654.GP2844@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAG48ez2gDDRtKaOcGdKLREd7RGtVzCypXiBMHBguOGSpxQFk3w@mail.gmail.com>

On Mon, Jan 06, 2020 at 05:39:30PM +0100, Jann Horn wrote:
> Hi!
> 
> I was chatting with kpsingh about BPF trampolines, and I noticed that
> it looks like BPF trampolines (as of current bpf-next/master) seem to
> be missing synchronization between trampoline code updates and
> trampoline execution. Or maybe I'm missing something?
> 
> If I understand correctly, trampolines are executed directly from the
> fentry placeholders at the start of arbitrary kernel functions, so
> they can run without any locks held. So for example, if task A starts
> executing a trampoline on entry to sys_open(), then gets preempted in
> the middle of the trampoline, and then task B quickly calls
> BPF_RAW_TRACEPOINT_OPEN twice, and then task A continues execution,
> task A will end up executing the middle of newly-written machine code,
> which can probably end up crashing the kernel somehow?
> 
> I think that at least to synchronize trampoline text freeing with
> concurrent trampoline execution, it is necessary to do something
> similar to what the livepatching code does with klp_check_stack(), and
> then either use a callback from the scheduler to periodically re-check
> tasks that were in the trampoline or let the trampoline tail-call into
> a cleanup helper that is part of normal kernel text. And you'd
> probably have to gate BPF trampolines on
> CONFIG_HAVE_RELIABLE_STACKTRACE.

ftrace uses synchronize_rcu_tasks() to flip between trampolines iirc.

  reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-06 16:39 Jann Horn
2020-01-06 16:56 ` Peter Zijlstra [this message]
2020-01-06 22:29   ` Alexei Starovoitov
2020-01-07  8:28   ` Petr Mladek
2020-01-07  9:03     ` Petr Mladek

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