linux-kernel - Re: [PATCH 26/26] x86, pkeys: Documentation

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Date:	Sun, 20 Sep 2015 10:55:54 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dave Hansen <dave@...1.net>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 26/26] x86, pkeys: Documentation

* Dave Hansen <dave@...1.net> wrote:

> +Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature
> +which will be found on future Intel CPUs.
> +
> +Memory Protection Keys provides a mechanism for enforcing page-based
> +protections, but without requiring modification of the page tables
> +when an application changes protection domains.  It works by
> +dedicating 4 previously ignored bits in each page table entry to a
> +"protection key", giving 16 possible keys.

Wondering how user-space is supposed to discover the number of protection keys,
is that CPUID leaf based, or hardcoded on the CPU feature bit?

> +There is also a new user-accessible register (PKRU) with two separate
> +bits (Access Disable and Write Disable) for each key.  Being a CPU
> +register, PKRU is inherently thread-local, potentially giving each
> +thread a different set of protections from every other thread.
> +
> +There are two new instructions (RDPKRU/WRPKRU) for reading and writing
> +to the new register.  The feature is only available in 64-bit mode,
> +even though there is theoretically space in the PAE PTEs.  These
> +permissions are enforced on data access only and have no effect on
> +instruction fetches.

Another question, related to enumeration as well: I'm wondering whether there's 
any way for the kernel to allocate a bit or two for its own purposes - such as 
protecting crypto keys? Or is the facility fundamentally intended for user-space 
use only?

Just a quick example: let's assume the kernel has an information leak hole, a way 
to read any kernel address and pass that to the kernel attacker. Let's also assume 
that the main crypto-keys of the kernel are protected by protection-keys. The code 
exposing the information leak will very likely have protection-key protected areas 
masked out, so the scope of the information leak is mitigated to a certain degree, 
the crypto keys are not readable.

Similarly, the pmem (persistent memory) driver could employ protection keys to 
keep terabytes of data 'masked out' most of the time - protecting data from kernel 
space memory corruption bugs.

Thanks,

	Ingo
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