linux-kernel - Re: [PATCH] hardening: add PROT_FINAL prot flag to mmap/mprotect

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Message-ID: <20120820180037.GV4232@outflux.net>
Date:	Mon, 20 Aug 2012 11:00:37 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Ard Biesheuvel <ard.biesheuvel@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hardening: add PROT_FINAL prot flag to mmap/mprotect

Hi,

On Tue, Aug 14, 2012 at 09:11:11PM +0200, Ard Biesheuvel wrote:
> This patch adds support for the PROT_FINAL flag to
> the mmap() and mprotect() syscalls.
> 
> The PROT_FINAL flag indicates that the requested set
> of protection bits should be final, i.e., it shall
> not be allowed for a subsequent mprotect call to
> set protection bits that were not set already.
> 
> This is mainly intended for the dynamic linker,
> which sets up the address space on behalf of
> dynamic binaries. By using this flag, it can
> prevent exploited code from remapping read-only
> executable code or data sections read-write.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...il.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

This seems like a good idea to me. It would allow more than just the
loader to harden userspace allocations. It's a more direct version of
PaX's "MPROTECT" feature[1]. That feature hardens existing loaders,
but doesn't play nice with JITs (like Java), but this lets a loader
(or JIT) opt-in to the protection and have some direct control over it.

It seems like there needs to be a sensible way to detect that this flag is
available, though.

-Kees

[1] http://pax.grsecurity.net/docs/mprotect.txt

-- 
Kees Cook                                            @outflux.net
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