Re: [PATCH 3/3] x86: kernel base offset ASLR

[Julien Tinnes]

On Thu, Apr 4, 2013 at 1:27 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 04/04/2013 01:23 PM, Julien Tinnes wrote:
>> On Thu, Apr 4, 2013 at 1:19 PM, Julien Tinnes <jln@xxxxxxxxxx> wrote:
>>> On Thu, Apr 4, 2013 at 1:12 PM, H. Peter Anvin <hpa@xxxxxxxxx> 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.
But yeah, this would give an incentive to plug more infoleaks. I'm not
sure what cost this would incur on kernel development.

There are by-design ones (printk) and bugs. I think we would want to
correct bugs regardless?
For by-design ones, privilege-reduction can often be an appropriate answer.

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.
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