From: Kees Cook <keescook@chromium.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Rik van Riel <riel@redhat.com>,
Daniel Micay <danielmicay@gmail.com>,
Qualys Security Advisory <qsa@qualys.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
"x86@kernel.org" <x86@kernel.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Dmitry Safonov <dsafonov@virtuozzo.com>,
Andy Lutomirski <luto@amacapital.net>,
Grzegorz Andrejczuk <grzegorz.andrejczuk@intel.com>,
Masahiro Yamada <yamada.masahiro@socionext.com>,
LKML <linux-kernel@vger.kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"kernel-hardening@lists.openwall.com"
<kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH v2] binfmt_elf: Use ELF_ET_DYN_BASE only for PIE
Date: Tue, 27 Jun 2017 13:08:39 -0700 [thread overview]
Message-ID: <CAGXu5jKaGjuq6s8XF+mctaL1Ckr2HtKgCnty2p6rUFQSmc-xHQ@mail.gmail.com> (raw)
In-Reply-To: <20170627144948.GD28078@dhcp22.suse.cz>
On Tue, Jun 27, 2017 at 7:49 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Wed 21-06-17 10:32:01, Kees Cook wrote:
>> 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.
>>
>> For 32-bit tasks when RLIMIT_STACK is set to RLIM_INFINITY, programs
>> are loaded below the mmap region. This means they can be made to collide
>> (CVE-2017-1000370) or nearly collide (CVE-2017-1000371) with pathological
>> stack regions. Lowering ELF_ET_DYN_BASE solves both by moving programs
>> above the mmap region in all cases, and will now additionally avoid
>> programs falling back to the mmap region by enforcing MAP_FIXED for
>> program loads (i.e. if it would have collided with the stack, now it
>> will fail to load instead of falling back to the mmap region).
>
> I do not understand this part. MAP_FIXED will simply unmap whatever
> was under the requested range, how it could help failing anything? So
> what would happen if something was mapped in that region, or is this
> impossible? Moreover MAP_FIXED close to stack will inhibit the stack gap
> protection.
Hmm, well, that's my misunderstanding. Regardless, it should still use
MAP_FIXED otherwise we end up with potentially unpredictable results.
(Note that MAP_FIXED is already used all all remaining allocations, it
was just missing on the first one.)
-Kees
--
Kees Cook
Pixel Security
WARNING: multiple messages have this Message-ID (diff)
From: Kees Cook <keescook@chromium.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Rik van Riel <riel@redhat.com>,
Daniel Micay <danielmicay@gmail.com>,
Qualys Security Advisory <qsa@qualys.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
"x86@kernel.org" <x86@kernel.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Dmitry Safonov <dsafonov@virtuozzo.com>,
Andy Lutomirski <luto@amacapital.net>,
Grzegorz Andrejczuk <grzegorz.andrejczuk@intel.com>,
Masahiro Yamada <yamada.masahiro@socionext.com>,
LKML <linux-kernel@vger.kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"kernel-hardening@lists.openwall.com"
<kernel-hardening@lists.openwall.com>
Subject: [kernel-hardening] Re: [PATCH v2] binfmt_elf: Use ELF_ET_DYN_BASE only for PIE
Date: Tue, 27 Jun 2017 13:08:39 -0700 [thread overview]
Message-ID: <CAGXu5jKaGjuq6s8XF+mctaL1Ckr2HtKgCnty2p6rUFQSmc-xHQ@mail.gmail.com> (raw)
In-Reply-To: <20170627144948.GD28078@dhcp22.suse.cz>
On Tue, Jun 27, 2017 at 7:49 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Wed 21-06-17 10:32:01, Kees Cook wrote:
>> 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.
>>
>> For 32-bit tasks when RLIMIT_STACK is set to RLIM_INFINITY, programs
>> are loaded below the mmap region. This means they can be made to collide
>> (CVE-2017-1000370) or nearly collide (CVE-2017-1000371) with pathological
>> stack regions. Lowering ELF_ET_DYN_BASE solves both by moving programs
>> above the mmap region in all cases, and will now additionally avoid
>> programs falling back to the mmap region by enforcing MAP_FIXED for
>> program loads (i.e. if it would have collided with the stack, now it
>> will fail to load instead of falling back to the mmap region).
>
> I do not understand this part. MAP_FIXED will simply unmap whatever
> was under the requested range, how it could help failing anything? So
> what would happen if something was mapped in that region, or is this
> impossible? Moreover MAP_FIXED close to stack will inhibit the stack gap
> protection.
Hmm, well, that's my misunderstanding. Regardless, it should still use
MAP_FIXED otherwise we end up with potentially unpredictable results.
(Note that MAP_FIXED is already used all all remaining allocations, it
was just missing on the first one.)
-Kees
--
Kees Cook
Pixel Security
next prev parent reply other threads:[~2017-06-27 20:08 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-21 17:32 [PATCH v2] binfmt_elf: Use ELF_ET_DYN_BASE only for PIE Kees Cook
2017-06-21 17:32 ` [kernel-hardening] " Kees Cook
2017-06-21 18:24 ` Rik van Riel
2017-06-21 18:24 ` [kernel-hardening] " Rik van Riel
2017-06-27 14:49 ` Michal Hocko
2017-06-27 14:49 ` [kernel-hardening] " Michal Hocko
2017-06-27 14:54 ` Daniel Micay
2017-06-27 14:54 ` [kernel-hardening] " Daniel Micay
2017-06-27 20:08 ` Kees Cook [this message]
2017-06-27 20:08 ` Kees Cook
2017-06-27 20:08 ` Kees Cook
2017-07-04 13:12 ` Michal Hocko
2017-07-04 13:12 ` [kernel-hardening] " Michal Hocko
2017-07-04 13:12 ` Michal Hocko
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