linux-kernel - Re: [PATCH v2] tracing, printk: Force no hashing when trace

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Date:   Tue, 3 Apr 2018 14:43:43 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v2] tracing, printk: Force no hashing when trace_printk()
 is used

On Tue, Apr 3, 2018 at 2:31 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> From: Steven Rostedt (VMware) <rostedt@...dmis.org>
>
> While debugging an issue I needed to see if the pointers were being
> processed correctly with trace_printk() and after using "%p" and
> triggering my bug and trace output, I was disappointed that all my
> pointers were random garbage and didn't produce anything useful for me.
> I had to rewrite all the trace_printk()s to use "%lx" instead.
>
> As trace_printk() is not to be used for anything but debugging, and
> this is enforced by printing in the dmesg:
>
>  **********************************************************
>  **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
>  **                                                      **
>  ** trace_printk() being used. Allocating extra memory.  **
>  **                                                      **
>  ** This means that this is a DEBUG kernel and it is     **
>  ** unsafe for production use.                           **
>  **                                                      **
>  ** If you see this message and you are not debugging    **
>  ** the kernel, report this immediately to your vendor!  **
>  **                                                      **
>  **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
>  **********************************************************
>
> on boot up if trace_printk() is used (or when a module is loaded that
> uses trace_printk()), we can safely assume that the use of
> trace_printk() is not going to be accidentally added to production code
> (and if it is, they should be whacked with an overcooked spaghetti
> noodle).
>
> A static_key is added called "trace_debug" and if it is set, then %p will
> not be hashed.

Hrm, well, using a static key does make it weirder for an attacker to enable. :)

Whatever the case, if you can get Linus's Ack, sure. I would expect
he'd want you to change all the trace_printk()s to %px with
justifications, though.

-Kees

-- 
Kees Cook
Pixel Security