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Date: Fri, 5 May 2017 10:23:50 +0200
From: Daniel Gruss <>
To: Thomas Garnier <>
CC: kernel list <>,
        Kernel Hardening
        Michael Schwarz
        Richard Fellner
        "Kirill A. Shutemov"
        Ingo Molnar <>,
        "" <>
Subject: Re: [RFC, PATCH] x86_64: KAISER - do not map
 kernel in user mode

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.

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