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Date:   Mon, 12 Nov 2018 09:53:29 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Masami Hiramatsu <mhiramat@kernel.org>,
        rostedt <rostedt@goodmis.org>
Cc:     Alexander Popov <alex.popov@linux.com>,
        kernel-hardening <kernel-hardening@lists.openwall.com>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Joerg Roedel <joro@8bytes.org>, Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>, Jan Kara <jack@suse.cz>,
        Dan Williams <dan.j.williams@intel.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        x86 <x86@kernel.org>, linux-kernel <linux-kernel@vger.kernel.org>
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Subject: Re: [PATCH 1/1] stackleak: Disable ftrace for stackleak.c
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Thread-Topic: stackleak: Disable ftrace for stackleak.c
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----- On Nov 11, 2018, at 9:50 PM, Masami Hiramatsu mhiramat@kernel.org wrote:

> Hi Alexander and Steve,
> 
> On Sun, 11 Nov 2018 20:53:51 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
>> On Sun, 11 Nov 2018 13:19:45 +0300
>> Alexander Popov <alex.popov@linux.com> wrote:
>> 
>> > On 11.11.2018 2:30, Steven Rostedt wrote:
>> > > On Sun, 11 Nov 2018 01:05:30 +0300
>> > > Alexander Popov <alex.popov@linux.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