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

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Message-ID: <CA+55aFxf3ExQEq2zhNhj4zk5nC5in9=1acVfynOVxZdN9RLbdA@mail.gmail.com>
Date:	Thu, 1 Oct 2015 21:38:10 -0400
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Hansen <dave@...1.net>
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 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.

No, what you *should* look at is to use the protection keys to
actually enforce a plain PROT_EXEC. That has never worked before
(because traditionally R implies X, and then we got NX).

That would at least allow people who know they don't intersperse
read-only constants in the code to use PROT_EXE only.

Of course, there may well be users who use PROT_EXE that actually *do*
do reads, and just relied on the old hardware behavior. So it's not
guaranteed to work either without any extra flags. But at least it's
worth a try, unlike the "yeah, the user asked for read, but the user
doesn't know what he's doing" thinking that is just crazy talk.

           Linus
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