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From: Daniel Micay <danielmicay@gmail.com>
To: Michal Hocko <mhocko@kernel.org>, Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Rik van Riel <riel@redhat.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, 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>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	kernel-hardening@lists.openwall.com
Subject: Re: [PATCH v2] binfmt_elf: Use ELF_ET_DYN_BASE only for PIE
Date: Tue, 27 Jun 2017 10:54:56 -0400	[thread overview]
Message-ID: <1498575296.1180.0.camel@gmail.com> (raw)
In-Reply-To: <20170627144948.GD28078@dhcp22.suse.cz>

On Tue, 2017-06-27 at 16:49 +0200, Michal Hocko 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.

I don't think there's a reason to use MAP_FIXED. PaX likely ignores the
address hint with RANDMMAP in that code, which would explain it there.

WARNING: multiple messages have this Message-ID (diff)
From: Daniel Micay <danielmicay@gmail.com>
To: Michal Hocko <mhocko@kernel.org>, Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Rik van Riel <riel@redhat.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, 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>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	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 10:54:56 -0400	[thread overview]
Message-ID: <1498575296.1180.0.camel@gmail.com> (raw)
In-Reply-To: <20170627144948.GD28078@dhcp22.suse.cz>

On Tue, 2017-06-27 at 16:49 +0200, Michal Hocko 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.

I don't think there's a reason to use MAP_FIXED. PaX likely ignores the
address hint with RANDMMAP in that code, which would explain it there.

  reply	other threads:[~2017-06-27 14:55 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 [this message]
2017-06-27 14:54     ` Daniel Micay
2017-06-27 20:08   ` Kees Cook
2017-06-27 20:08     ` [kernel-hardening] " 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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