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

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Date:   Mon, 12 Nov 2018 11:50:58 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Alexander Popov <alex.popov@...ux.com>,
        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>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Masami Hiramatsu <mhiramat@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] stackleak: Disable ftrace for stackleak.c

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. 

Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>