Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map kernel in user mode

[Daniel Gruss]

On 05.05.2017 10:23, Daniel Gruss wrote:
> >  - How this approach prevent the hardware attacks you mentioned? You
> > still have to keep a part of _text in the pagetable and an attacker
> > could discover it no? (and deduce the kernel base address).
>
> These parts are moved to a different section (.user_mapped) which is at a possibly predictable location - the location
> of the randomized parts of the kernel is independent of the location of .user_mapped.
> The code/data footprint for .user_mapped is quite small, helping to reduce or eliminate the attack surface...

We just discussed that in our group again: although we experimented with this part, it's not yet included in the patch. 
The solution we sketched is, as I wrote, we map the required (per-thread) variables in the user CR3 to a fixed location 
in memory. During the context switch, only this fixed part remains mapped but not the randomized pages. This is not a 
lot of work, because it's just mapping a few more pages and fixing a 1 or 2 lines in the context switch.