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

From: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
To: Thomas Garnier <thgarnie@google.com>
Cc: kernel list <linux-kernel@vger.kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	"clementine.maurice@iaik.tugraz.at" 
	<clementine.maurice@iaik.tugraz.at>,
	"moritz.lipp@iaik.tugraz.at" <moritz.lipp@iaik.tugraz.at>,
	Michael Schwarz <michael.schwarz@iaik.tugraz.at>,
	Richard Fellner <richard.fellner@student.tugraz.at>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Ingo Molnar <mingo@kernel.org>,
	"anders.fogh@gdata-adan.de" <anders.fogh@gdata-adan.de>
Subject: Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map kernel in user mode
Date: Fri, 5 May 2017 10:23:50 +0200	[thread overview]
Message-ID: <55fa194e-69bd-fe39-f915-6f77673aea36@iaik.tugraz.at> (raw)
In-Reply-To: <CAJcbSZHcAi37FfFBKF4MsC3WM-RY5ar_e1pgMScb=S3+_-wqVQ@mail.gmail.com>

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.