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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()")
Date: Sat, 30 Jan 2021 21:10:22 +0900
Message-ID: <20210130211022.d64c4caaf6667ec70a871420@kernel.org> (raw)
In-Reply-To: <20210130030840.hodq2ixpkdoue5jd@ast-mbp.dhcp.thefacebook.com>

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>

      reply index

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <25cd2608-03c2-94b8-7760-9de9935fde64@suse.com>
     [not found] ` <20210128001353.66e7171b395473ef992d6991@kernel.org>
     [not found]   ` <20210128002452.a79714c236b69ab9acfa986c@kernel.org>
     [not found]     ` <a35a6f15-9ab1-917c-d443-23d3e78f2d73@suse.com>
     [not found]       ` <20210128103415.d90be51ec607bb6123b2843c@kernel.org>
2021-01-28  3:38         ` Masami Hiramatsu
2021-01-28  7:11           ` Nikolay Borisov
2021-01-28 16:12           ` Nikolay Borisov
2021-01-28 16:45             ` Nikolay Borisov
2021-01-28 16:50               ` Josh Poimboeuf
2021-01-28 21:52                 ` [PATCH] x86: Disable CET instrumentation in the kernel Josh Poimboeuf
2021-01-29  6:23                   ` Nikolay Borisov
2021-01-29 10:21                   ` Borislav Petkov
     [not found]                     ` <20210129151034.iba4eaa2fuxsipqa@treble>
2021-01-29 16:30                       ` Borislav Petkov
2021-01-29 16:49                         ` Josh Poimboeuf
2021-01-29 16:54                           ` Nikolay Borisov
2021-01-29 17:03                             ` Josh Poimboeuf
2021-01-29 17:07                               ` Borislav Petkov
2021-01-29 17:58                                 ` Seth Forshee
2021-01-28 18:24               ` kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()") Peter Zijlstra
2021-01-29  1:34                 ` Alexei Starovoitov
2021-01-29  6:36                   ` Nikolay Borisov
     [not found]                   ` <YBPNyRyrkzw2echi@hirez.programming.kicks-ass.net>
     [not found]                     ` <20210129224011.81bcdb3eba1227c414e69e1f@kernel.org>
     [not found]                       ` <20210129105952.74dc8464@gandalf.local.home>
2021-01-29 16:24                         ` Peter Zijlstra
2021-01-29 17:45                           ` Alexei Starovoitov
2021-01-29 17:59                             ` Peter Zijlstra
2021-01-29 19:01                               ` Steven Rostedt
2021-01-29 21:05                                 ` Alexei Starovoitov
2021-01-30  1:41                                   ` Masami Hiramatsu
2021-01-29 21:24                                 ` Steven Rostedt
2021-01-30  8:28                                   ` Peter Zijlstra
2021-01-30 12:44                                     ` Steven Rostedt
2021-02-02 10:45                                       ` Peter Zijlstra
2021-02-02 14:52                                         ` Steven Rostedt
2021-02-02 16:45                                           ` Peter Zijlstra
2021-02-02 16:56                                             ` Steven Rostedt
2021-02-02 18:30                                               ` Peter Zijlstra
2021-02-02 21:05                                                 ` Steven Rostedt
2021-02-03 13:33                                                   ` Masami Hiramatsu
2021-02-03 13:52                                                     ` Steven Rostedt
2021-01-30  2:02                               ` Masami Hiramatsu
2021-01-30  3:08                                 ` Alexei Starovoitov
2021-01-30 12:10                                   ` Masami Hiramatsu [this message]

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