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Date:   Sat, 30 Jan 2021 21:10:22 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Nikolay Borisov <nborisov@suse.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Alexei Starovoitov <ast@kernel.org>, bpf <bpf@vger.kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: kprobes broken since 0d00449c7a28
 ("x86: Replace ist_enter() with nmi_enter()")
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On Fri, 29 Jan 2021 19:08:40 -0800
Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> On Sat, Jan 30, 2021 at 11:02:49AM +0900, Masami Hiramatsu wrote:
> > On Fri, 29 Jan 2021 18:59:43 +0100
> > Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > > On Fri, Jan 29, 2021 at 09:45:48AM -0800, Alexei Starovoitov wrote:
> > > > Same things apply to bpf side. We can statically prove safety for
> > > > ftrace and kprobe attaching whereas to deal with NMI situation we
> > > > have to use run-time checks for recursion prevention, etc.
> > > 
> > > I have no idea what you're saying. You can attach to functions that are
> > > called with random locks held, you can create kprobes in some very
> > > sensitive places.
> > > 
> > > What can you staticlly prove about that?
> > 
> > For the bpf and the kprobe tracer, if a probe hits in the NMI context,
> > it can call the handler with another handler processing events.
> > 
> > kprobes is carefully avoiding the deadlock by checking recursion
> > with per-cpu variable. But if the handler is shared with the other events
> > like tracepoints, it needs to its own recursion cheker too.
> > 
> > So, Alexei, maybe you need something like this instead of in_nmi() check.
> > 
> > DEFINE_PER_CPU(bool, under_running_bpf);
> > 
> > common_handler()
> > {
> > 	if (__this_cpu_read(under_running_bpf))
> > 		return;
> > 	__this_cpu_write(under_running_bpf, true);
> > 	/* execute bpf prog */
> > 	__this_cpu_write(under_running_bpf, false);	
> > }
> > 
> > Does this work for you?
> 
> This exactly check is already in trace_call_bpf.
> Right after if (in_nmi()).
> See bpf_prog_active. It serves different purpose though.
> Simply removing if (in_nmi()) from trace_call_bpf is a bit scary.
> I need to analyze all code paths first.

OK, if bpf already avoids its recursion, other considerable case is
that some resources are shared among bpf_prog and other parts. Since
asynchronous NMI can occur anywhere, such resource usage can conflict
with bpf_prog.

Kprobes had similar issue, so I set a dummy kprobes to current_kprobes
for protecting such critical sections.
See kprobe_busy_begin()/end() and where those are used.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>