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Date:   Wed, 29 Nov 2017 10:45:43 -0700
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>, linux-api@...r.kernel.org
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
        Florian Weimer <fweimer@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Michal Hocko <mhocko@...e.com>,
        Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
        Joel Stanley <joel@....id.au>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 2/2] fs, elf: drop MAP_FIXED usage from elf_map

On 11/29/2017 07:42 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> Both load_elf_interp and load_elf_binary rely on elf_map to map segments
> on a controlled address and they use MAP_FIXED to enforce that. This is
> however dangerous thing prone to silent data corruption which can be
> even exploitable. Let's take CVE-2017-1000253 as an example. At the time
> (before eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE"))
> ELF_ET_DYN_BASE was at TASK_SIZE / 3 * 2 which is not that far away from
> the stack top on 32b (legacy) memory layout (only 1GB away). Therefore
> we could end up mapping over the existing stack with some luck.
> 
> The issue has been fixed since then (a87938b2e246 ("fs/binfmt_elf.c:
> fix bug in loading of PIE binaries")), ELF_ET_DYN_BASE moved moved much
> further from the stack (eab09532d400 and later by c715b72c1ba4 ("mm:
> revert x86_64 and arm64 ELF_ET_DYN_BASE base changes")) and excessive
> stack consumption early during execve fully stopped by da029c11e6b1
> ("exec: Limit arg stack to at most 75% of _STK_LIM"). So we should be
> safe and any attack should be impractical. On the other hand this is
> just too subtle assumption so it can break quite easily and hard to
> spot.
> 
> I believe that the MAP_FIXED usage in load_elf_binary (et. al) is still
> fundamentally dangerous. Moreover it shouldn't be even needed. We are
> at the early process stage and so there shouldn't be unrelated mappings
> (except for stack and loader) existing so mmap for a given address
> should succeed even without MAP_FIXED. Something is terribly wrong if
> this is not the case and we should rather fail than silently corrupt the
> underlying mapping.
> 
> Address this issue by changing MAP_FIXED to the newly added
> MAP_FIXED_SAFE. This will mean that mmap will fail if there is an
> existing mapping clashing with the requested one without clobbering it.
> 
> Cc: Abdul Haleem <abdhalee@...ux.vnet.ibm.com>
> Cc: Joel Stanley <joel@....id.au>
> Acked-by: Kees Cook <keescook@...omium.org>
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---

Reviewed-by: Khalid Aziz <khalid.aziz@...cle.com>

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