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Message-ID: <20130405070530.GA26889@gmail.com>
Date: Fri, 5 Apr 2013 09:05:30 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Julien Tinnes <jln@...gle.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>,
linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
Jarkko Sakkinen <jarkko.sakkinen@...el.com>,
Matthew Garrett <mjg@...hat.com>,
Matt Fleming <matt.fleming@...el.com>,
Eric Northup <digitaleric@...gle.com>,
Dan Rosenberg <drosenberg@...curity.com>,
Will Drewry <wad@...omium.org>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3/3] x86: kernel base offset ASLR
* Julien Tinnes <jln@...gle.com> wrote:
> On Thu, Apr 4, 2013 at 1:27 PM, H. Peter Anvin <hpa@...or.com> wrote:
> > On 04/04/2013 01:23 PM, Julien Tinnes wrote:
> >> On Thu, Apr 4, 2013 at 1:19 PM, Julien Tinnes <jln@...gle.com> wrote:
> >>> On Thu, Apr 4, 2013 at 1:12 PM, H. Peter Anvin <hpa@...or.com> wrote:
> >>>> On 04/04/2013 01:07 PM, Kees Cook wrote:
> >>>>> However, the benefits of
> >>>>> this feature in certain environments exceed the perceived weaknesses[2].
> >>>>
> >>>> Could you clarify?
> >>>
> >>> I think privilege reduction in general, and sandboxing in particular,
> >>> can make KASLR even more useful. A lot of the information leaks can be
> >>> mitigated in the same way as attack surface and vulnerabilities can be
> >>> mitigated.
> >>
> >> Case in point:
> >> - leaks of 64 bits kernel values to userland in compatibility
> >> sub-mode. Sandboxing by using seccomp-bpf can restrict a process to
> >> the 64-bit mode API.
> >> - restricting access to the syslog() system call
> >>
> >
> > That doesn't really speak to the value proposition. My concern is that
> > we're going to spend a lot of time chasing/plugging infoleaks instead of
> > tackling bigger problems.
>
> Certain leaks are already an issue, even without kernel base
> randomization.
Definitely. Stealth infiltration needs a high reliability expoit,
especially if the attack vector used is a zero day kernel vulnerability.
Injecting uncertainty gives us a chance to get a crash logged and the
vulnerability exposed.
> But yeah, this would give an incentive to plug more infoleaks. I'm not
> sure what cost this would incur on kernel development.
I consider it a plus on kernel development - the more incentives to plug
infoleaks, the better.
> There are by-design ones (printk) and bugs. I think we would want to
> correct bugs regardless?
Definitely.
> For by-design ones, privilege-reduction can often be an appropriate answer.
Correct, that's the motivation behind kptr_restrict and dmsg_restrict.
> I really see KASLR as the next natural step:
>
> 1. Enforce different privilege levels via the kernel
> 2. Attackers attack the kernel directly
> 3a. Allow user-land to restrict the kernel's attack surface and
> develop sandboxes (seccomp-bpf, kvm..)
> 3b. Add more exploitation defenses to the kernel, leveraging (3a) and (1).
>
> > 8 bits of entropy is not a lot.
>
> It would certainly be nice to have more, but it's a good first start.
> Unlike user-land segfaults, many kernel-mode panics aren't recoverable
> for an attacker.
The other aspect of even just a couple of bits of extra entropy is that it
changes the economics of worms and other remote attacks: there's a
significant difference between being able to infect one machine per packet
and only 1 out of 256 machines while the other 255 get crashed.
The downside is debuggability - so things like 'debug' on the kernel boot
command line should probably disable this feature automatically.
Thanks,
Ingo
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