linux-kernel - Re: [RFC][PATCH] tracing, printk: Force no hashing when trace

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Date:   Wed, 4 Apr 2018 12:52:37 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Tobin C. Harding" <me@...in.cc>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Laight <David.Laight@...lab.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [RFC][PATCH] tracing, printk: Force no hashing when
 trace_printk() is used

On Wed, 4 Apr 2018 09:27:10 -0700
Kees Cook <keescook@...omium.org> wrote:

> On Wed, Apr 4, 2018 at 12:49 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Tue, Apr 03, 2018 at 05:06:12PM -0400, Steven Rostedt wrote:  
> >> If you are concerned about attack surface, I could make it a bit more
> >> difficult to tweak by malicious software. What about the patch below?
> >> It would be much more difficult to modify this knob from an attack
> >> vector.  
> >
> > Not if you build using clang, because that doesn't support asm-goto and
> > thus falls back to a simple runtime variable, which is exactly what Kees
> > didn't want.  
> 
> Nah, Clang will get asm-goto soon. I'm not worried about that. Besides
> the "yay new target issue", I was concerned about Linus yelling about
> seeing the knob added he specifically said he didn't want. :P
>

This isn't really a knob. It can only get enabled when trace_printk()
is added, which requires a change to the kernel source code. It's just
an easy way on doing it yourself and recompiling.

No userspace can enable it, with the exception of adding a kernel
module that has a trace_printk() in it. A kernel command line option
wont even enable it.

-- Steve