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

[Daniel Gruss]

On 04.05.2017 17:28, Thomas Garnier wrote:
> Please read the documentation on submitting patches [1] and coding style [2].

I will have a closer look at that.

>  - 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...

> You also need to make it clear that btb attacks are still possible.

By just increasing the KASLR randomization range, btb attacks can be 
mitigated (for free).

>  - What is the perf impact?

It will vary for different machines. We have promising results (<1%) for 
an i7-6700K with representative benchmarks. However, for older systems 
or for workloads with a lot of pressure on some TLB levels, the 
performance may be much worse.