Re: [PATCH 00/18] arm64: Unmap the kernel whilst running in userspace (KAISER)

[Ard Biesheuvel]

On 22 November 2017 at 16:19, Pavel Machek <pavel@xxxxxx> wrote:
> Hi!
>
>> This patch series implements something along the lines of KAISER for arm64:
>>
>>   https://gruss.cc/files/kaiser.pdf
>>
>> although I wrote this from scratch because the paper has some funny
>> assumptions about how the architecture works. There is a patch series
>> in review for x86, which follows a similar approach:
>>
>>   http://lkml.kernel.org/r/<20171110193058.BECA7D88@xxxxxxxxxxxxxxxxxx>
>>
>> and the topic was recently covered by LWN (currently subscriber-only):
>>
>>   https://lwn.net/Articles/738975/
>>
>> The basic idea is that transitions to and from userspace are proxied
>> through a trampoline page which is mapped into a separate page table and
>> can switch the full kernel mapping in and out on exception entry and
>> exit respectively. This is a valuable defence against various KASLR and
>> timing attacks, particularly as the trampoline page is at a fixed virtual
>> address and therefore the kernel text can be randomized
>> independently.
>
> If I'm willing to do timing attacks to defeat KASLR... what prevents
> me from using CPU caches to do that?
>

Because it is impossible to get a cache hit on an access to an unmapped address?

> There was blackhat talk about exactly that IIRC...
>                                                                         Pavel
> --
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html