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

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Date:   Fri, 26 Aug 2016 08:30:16 -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>,
        the arch/x86 maintainers <x86@...nel.org>,
        Nilay Vaish <nilayvaish@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH 3/6] x86/dumpstack: make printk_stack_address() more
 generally useful

On Thu, Aug 25, 2016 at 11:12:40PM -0700, Linus Torvalds wrote:
> On Aug 25, 2016 10:57 PM, "Josh Poimboeuf" <jpoimboe@...hat.com> wrote:
> >
> > 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?
> 
> The point is: kptr_restrict actually makes sense, and is widely useful. It
> doesn't really end up hurting normal things. It's a pretty targeted thing,
> and generally doesn't actually hurt. You can still do basic health
> monitoring without having to get elevated privileges, for example.
> 
> Even system maintainers don't want to be root all the time. In fact, I
> suspect that the better a system maintainer you are, the less you want to
> be root - but you'll still want to see logs etc.
> 
> So note the difference between kptr_restrict and dmesg_restrict.
> 
> One is useful in a pretty wide environment, the other simply is not.
> 
> > So what's the difference between expecting the user to set kptr_restrict
> > vs dmesg_restrict?
> 
> Do you see the difference now?
> 
> kptr_restrict simply doesn't hurt as much as dmesg_restrict, so you can
> enable it fairly widely by default.
> 
> That makes it the *much* better security option. Because security options
> that you can't enable aren't actually useful.

Yeah, at least for human-administered systems, that does make sense.
Grumpy sysadmins don't want to type "sudo dmesg" or "sudo journalctl"
because a) they don't like change; and b) using sudo adds risk.

And a security option which is never used is indeed useless.  So *maybe*
that's a good enough argument for expecting the user to only enable
kptr_restrict instead of both.

But with cloud, devops, mobile, embedded, IoT, [insert buzzword], it
seems most systems are actually managed by software nowadays.  Then the
above arguments don't seem to apply, and dmesg_restrict could still be
quite widely useful, no?

-- 
Josh