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Date:   Wed, 29 Nov 2017 10:45:43 -0700
From:   Khalid Aziz <>
To:     Michal Hocko <>,
Cc:     Michael Ellerman <>,
        Andrew Morton <>,
        Russell King - ARM Linux <>,
        Andrea Arcangeli <>,,
        LKML <>,,
        Florian Weimer <>,
        John Hubbard <>,
        Michal Hocko <>,
        Abdul Haleem <>,
        Joel Stanley <>,
        Kees Cook <>
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 <>
> 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 <>
> Cc: Joel Stanley <>
> Acked-by: Kees Cook <>
> Signed-off-by: Michal Hocko <>
> ---

Reviewed-by: Khalid Aziz <>

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