• Michal Hocko's avatar
    fs, elf: drop MAP_FIXED usage from elf_map · 4ed28639
    Michal Hocko authored
    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 commit
    eab09532: "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 (a87938b2: "fs/binfmt_elf.c: fix
    bug in loading of PIE binaries"), ELF_ET_DYN_BASE moved moved much
    further from the stack (eab09532 and later by c715b72c: "mm:
    revert x86_64 and arm64 ELF_ET_DYN_BASE base changes") and excessive
    stack consumption early during execve fully stopped by da029c11
    ("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_NOREPLACE.  This will mean that mmap will fail if there is an
    existing mapping clashing with the requested one without clobbering it.
    
    [mhocko@suse.com: fix build]
    [akpm@linux-foundation.org: coding-style fixes]
    [avagin@openvz.org: don't use the same value for MAP_FIXED_NOREPLACE and MAP_SYNC]
      Link: http://lkml.kernel.org/r/20171218184916.24445-1-avagin@openvz.org
    Link: http://lkml.kernel.org/r/20171213092550.2774-3-mhocko@kernel.orgSigned-off-by: default avatarMichal Hocko <mhocko@suse.com>
    Signed-off-by: default avatarAndrei Vagin <avagin@openvz.org>
    Signed-off-by: default avatarMichal Hocko <mhocko@suse.com>
    Reviewed-by: default avatarKhalid Aziz <khalid.aziz@oracle.com>
    Acked-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Acked-by: default avatarKees Cook <keescook@chromium.org>
    Cc: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
    Cc: Joel Stanley <joel@jms.id.au>
    Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    4ed28639
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