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

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Message-ID: <560EC81B.60809@sr71.net>
Date:	Fri, 2 Oct 2015 11:08:27 -0700
From:	Dave Hansen <dave@...1.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Kees Cook <keescook@...gle.com>, Ingo Molnar <mingo@...nel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.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

On 10/01/2015 06:38 PM, Linus Torvalds wrote:
> On Thu, Oct 1, 2015 at 6:56 PM, Dave Hansen <dave@...1.net> wrote:
>>
>> Also, a quick ftrace showed that most mmap() callers that set PROT_EXEC
>> also set PROT_READ.  I'm just assuming that folks are setting PROT_READ
>> but aren't _really_ going to read it, so we can safely deny them all
>> access other than exec.
> 
> That's a completely insane assumption. There are tons of reasons to
> have code and read-only data in the same segment, and it's very
> traditional. Just assuming that you only execute out of something that
> has PROT_EXEC | PROT_READ is insane.

Yes, it's insane, and I confirmed that ld.so actually reads some stuff
out of the first page of the r-x part of the executable.

But, it did find a bug in my code where I wouldn't allow instruction
fetches to fault in pages in a pkey-protected area, so it wasn't a
completely worthless exercise.
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