linux-kernel - Re: [PATCH 3/6] x86/dumpstack: make printk_stack

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Date:   Fri, 26 Aug 2016 00:56:58 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Nilay Vaish <nilayvaish@...il.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 3/6] x86/dumpstack: make printk_stack_address() more
 generally useful

On Thu, Aug 25, 2016 at 09:40:12PM -0700, Linus Torvalds wrote:
> On Thu, Aug 25, 2016 at 8:19 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > So yes, dmesg_restrict sounds useful to me.  It's a way to prevent users
> > from seeing kernel addresses without affecting my ability to debug
> > issues.  For a locked down system, why would non-root users need to
> > access dmesg anyway?
> 
> That's the point. It is only useful for locked-down systems.
> 
> But that also means that IT IS NOT USEFUL AS A SECURITY ARGUMENT -
> since it's simply not relevant to most systems out there.
> 
> Most systems aren't locked down.

Ok, so maybe removing kernel text addresses from the stack dump wouldn't
be the end of the world.

But I still don't quite understand your statement that dmesg_restrict is
only useful for locked down systems.

To prevent kernel address disclosure, it seems we already rely on the
user setting kptr_restrict today, otherwise I can do cat
/proc/self/stack and the game is already lost, right?

So what's the difference between expecting the user to set kptr_restrict
vs dmesg_restrict?

-- 
Josh