linux-kernel - Re: [PATCH v2 0/5] Use ELF_ET_DYN

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Message-ID: <CAGXu5jKPu3NS5i3eAa6QhMHE_r6Z6U7d5bGsNfp25N3=04hszQ@mail.gmail.com>
Date:   Sat, 24 Jun 2017 06:58:59 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Daniel Micay <danielmicay@...il.com>,
        Qualys Security Advisory <qsa@...lys.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Pratyush Anand <panand@...hat.com>,
        James Hogan <james.hogan@...tec.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Grzegorz Andrejczuk <grzegorz.andrejczuk@...el.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2 0/5] Use ELF_ET_DYN_BASE only for PIE

On Sat, Jun 24, 2017 at 2:11 AM, Russell King - ARM Linux
<linux@...linux.org.uk> wrote:
> On Fri, Jun 23, 2017 at 01:59:55PM -0700, Kees Cook wrote:
>> This is v2 (to refresh the 5 patches in -mm) for moving ELF_ET_DYN_BASE
>> safely lower. Changes are clarifications in the commit logs (suggested
>> by mpe), a compat think-o fix for arm64 (thanks to Ard), and to add
>> Rik and mpe's Acks.
>>
>> Quoting patch 1/5:
>>
>> The ELF_ET_DYN_BASE position was originally intended to keep loaders
>> away from ET_EXEC binaries. (For example, running "/lib/ld-linux.so.2
>> /bin/cat" might cause the subsequent load of /bin/cat into where the
>> loader had been loaded.) With the advent of PIE (ET_DYN binaries with
>> an INTERP Program Header), ELF_ET_DYN_BASE continued to be used since
>> the kernel was only looking at ET_DYN. However, since ELF_ET_DYN_BASE
>> is traditionally set at the top 1/3rd of the TASK_SIZE, a substantial
>> portion of the address space is unused.
>
> With existing kernels on ARM:
>
> 00010000-00017000 r-xp 00000000 08:01 270810     /bin/cat
> 00026000-00027000 r--p 00006000 08:01 270810     /bin/cat
> 00027000-00028000 rw-p 00007000 08:01 270810     /bin/cat
> 7f661000-7f679000 r-xp 00000000 08:01 281659     /lib/arm-linux-gnueabihf/ld-2.23.so
> 7f688000-7f689000 r--p 00017000 08:01 281659     /lib/arm-linux-gnueabihf/ld-2.23.so
> 7f689000-7f68a000 rw-p 00018000 08:01 281659     /lib/arm-linux-gnueabihf/ld-2.23.so
>
> If the loader is loaded at 4MB, this means the size of an ET_EXEC
> program is limited to less than 4MB - and distros aren't yet
> building everything as PIE on ARM.

The loader isn't loaded at 4MB; that's what patch 1 changes: loaders
are moved into the mmap region so they will not collide with either
ET_EXEC nor PIE (ET_DYN-with-INTERP).

(After this patch, the name "ELF_ET_DYN_BASE" becomes a bit misleading...)

-Kees

-- 
Kees Cook
Pixel Security