kernel-hardening - Re: [PATCH 1/1] stackleak: Disable ftrace for stackleak.c

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Message-ID: <97842534.3842.1542034409326.JavaMail.zimbra@efficios.com>
Date: Mon, 12 Nov 2018 09:53:29 -0500 (EST)
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Masami Hiramatsu <mhiramat@...nel.org>, rostedt <rostedt@...dmis.org>
Cc: Alexander Popov <alex.popov@...ux.com>, 
	kernel-hardening <kernel-hardening@...ts.openwall.com>, 
	Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>, 
	Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>, 
	Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>, 
	Thomas Gleixner <tglx@...utronix.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, 
	Peter Zijlstra <peterz@...radead.org>, Jan Kara <jack@...e.cz>, 
	Dan Williams <dan.j.williams@...el.com>, 
	Masahiro Yamada <yamada.masahiro@...ionext.com>, 
	x86 <x86@...nel.org>, linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] stackleak: Disable ftrace for stackleak.c

----- On Nov 11, 2018, at 9:50 PM, Masami Hiramatsu mhiramat@...nel.org wrote:

> Hi Alexander and Steve,
> 
> On Sun, 11 Nov 2018 20:53:51 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
>> On Sun, 11 Nov 2018 13:19:45 +0300
>> Alexander Popov <alex.popov@...ux.com> wrote:
>> 
>> > On 11.11.2018 2:30, Steven Rostedt wrote:
>> > > On Sun, 11 Nov 2018 01:05:30 +0300
>> > > Alexander Popov <alex.popov@...ux.com> wrote:
>> > >   
>> > >> The stackleak_erase() function is called on the trampoline stack at the
>> > >> end of syscall. This stack is not big enough for ftrace operations,
>> > >> e.g. it can be overflowed if we enable kprobe_events for stackleak_erase().
>> > > 
>> > > Is the issue with kprobes or with function tracing? Because this stops
>> > > function tracing which I only want disabled if function tracing itself
>> > > is an issue, not for other things that may use the function tracing
>> > > infrastructure.
>> > 
>> > Hello Steven,
>> > 
>> > I believe that stackleak erasing is not compatible with function tracing itself.
>> > That's what the kernel testing robot has hit:
>> > https://www.openwall.com/lists/kernel-hardening/2018/11/09/1
>> > 
>> > I used kprobe_events just to reproduce the problem:
>> > https://www.openwall.com/lists/kernel-hardening/2018/11/09/4
>> 
>> Have you tried adding a "notrace" to stackleak_erase()?
>> 
>> Not tracing the entire file is a bit of overkill. There's no reason
>> ftrace can't trace stack_erasing_sysctl() or perhaps even
>> stackleak_track_stack() as that may be very interesting to trace.
> 
> I think it is not enough for stopping kprobes. If you want to stop the kprobes
> (int3 version) on stackleak_erase(), you should use
> NOKPROBE_SYMBOL(stackleak_erase),
> since kprobes can work without ftrace.

Just to clarify: AFAIU you guys are recommending to add _both_ a "notrace"
annotation to stackleak_erase() _and_ a NOKPROBE_SYMBOL(stackleak_erase),
so neither function tracing nor kprobes can hook on that function.

Thanks,

Mathieu

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

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