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

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Date:	Thu, 24 Sep 2015 12:10:00 -0700
From:	Dave Hansen <dave@...1.net>
To:	Ingo Molnar <mingo@...nel.org>
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>, Kees Cook <keescook@...gle.com>
Subject: Re: [PATCH 26/26] x86, pkeys: Documentation

On 09/24/2015 02:49 AM, Ingo Molnar wrote:
> * Dave Hansen <dave@...1.net> wrote:
>>> 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?
>>
>> No, that's not possible with the current setup.
> 
> Ok, then another question, have you considered the following usecase:
> 
> AFAICS pkeys only affect data loads and stores. Instruction fetches are notably 
> absent from the documentation. Can you clarify that instructions can be fetched 
> and executed from PTE_READ but pkeys-all-access-disabled pags?

That is my understanding.  I don't have a test for it, but I'll go make one.

> If yes then this could be a significant security feature / usecase for pkeys: 
> executable sections of shared libraries and binaries could be mapped with pkey 
> access disabled. If I read the Intel documentation correctly then that should be 
> possible.

Agreed.  I've even heard from some researchers who are interested in this:

https://www.infsec.cs.uni-saarland.de/wp-content/uploads/sites/2/2014/10/nuernberger2014ccs_disclosure.pdf

> I.e. AFAICS pkeys could be used to create true '--x' permissions for executable 
> (user-space) pages.

Just remember that all of the protections are dependent on the contents
of PKRU.  If an attacker controls the Access-Disable bit in PKRU for the
executable-only region, you're sunk.

But, that either requires being able to construct and execute arbitrary
code *or* call existing code that sets PKRU to the desired values.
Which, I guess, gets harder to do if all of the the wrpkru's are *in*
the execute-only area.

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