Re: [PATCH 26/26] x86, pkeys: Documentation

From: Dave Hansen
Date: Thu Sep 24 2015 - 15:10:12 EST

On 09/24/2015 02:49 AM, Ingo Molnar wrote:
> * Dave Hansen <dave@xxxxxxxx> 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:

> 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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