linux-kernel - Re: [PATCH 31/31] x86, pkeys: execute-only support

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Date:	Thu, 7 Jan 2016 14:13:11 -0800
From:	Dave Hansen <dave@...1.net>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, X86 ML <x86@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...gle.com>
Subject: Re: [PATCH 31/31] x86, pkeys: execute-only support

On 01/07/2016 01:10 PM, Andy Lutomirski wrote:
> On Wed, Jan 6, 2016 at 4:01 PM, Dave Hansen <dave@...1.net> wrote:
>> From: Dave Hansen <dave.hansen@...ux.intel.com>
>> Protection keys provide new page-based protection in hardware.
>> But, they have an interesting attribute: they only affect data
>> accesses and never affect instruction fetches.  That means that
>> if we set up some memory which is set as "access-disabled" via
>> protection keys, we can still execute from it.
>> could lose the bits in PKRU that enforce execute-only
>> permissions.  To avoid this, we suggest avoiding ever calling
>> mmap() or mprotect() when the PKRU value is expected to be
>> stable.
> 
> This may be a bit unfortunate for people who call mmap from signal
> handlers.  Admittedly, the failure mode isn't that bad.

mmap() isn't in the list of async-signal-safe functions, so it's bad
already.

> Out of curiosity, do you have timing information for WRPKRU and
> RDPKRU?  If they're fast and if anyone ever implements my deferred
> xstate restore idea, then the performance issue goes away and we can
> stop caring about whether PKRU is in the init state.

I don't have timing information that I can share.  From my perspective,
they're pretty fast, *not* like an MSR write or something.  I think
they're fast enough to use in the context switch path.  I'd say PKRU is
in XSAVE for consistency more than for performance.