Subject: Re: [PATCH 31/32] x86, pkeys: execute-only support
To: Kees Cook <keescook@google.com>
References: <20151214190542.39C4886D@viggo.jf.intel.com>
 <20151214190632.6A741188@viggo.jf.intel.com>
 <CAGXu5jJ5oHy11Uy4N2m1aa2A9ar9-oH_kez9jq=gM8CVSj734Q@mail.gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>,
        "x86@kernel.org" <x86@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Dave Hansen <dave.hansen@linux.intel.com>
From: Dave Hansen <dave@sr71.net>
Message-ID: <566F52CE.6080501@sr71.net>
Date: Mon, 14 Dec 2015 15:37:50 -0800
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On 12/14/2015 12:05 PM, Kees Cook wrote:
> On Mon, Dec 14, 2015 at 11:06 AM, Dave Hansen <dave@sr71.net> wrote:
>> > From: Dave Hansen <dave.hansen@linux.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.
...
>> > I haven't found any userspace that does this today.
> To realistically take advantage of this, it sounds like the linker
> would need to know to keep bss and data page-aligned away from text,
> and then set text to PROT_EXEC only?
> 
> Do you have any example linker scripts for this?

Nope.  My linker-fu is weak.

Can we even depend on the linker by itself?  Even if the sections were
marked --x, we can't actually use them with those permissions unless we
have protection keys.

Do we need some special tag on the section to tell the linker to map it
as --x under some conditions and r-x for others?

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