elf: enforce MAP_FIXED on overlaying elf segments

Anshuman has reported that with "fs, elf: drop MAP_FIXED usage from
elf_map" applied, some ELF binaries in his environment fail to start
with

 [   23.423642] 9148 (sed): Uhuuh, elf segment at 0000000010030000 requested but the memory is mapped already
 [   23.423706] requested [10030000, 10040000] mapped [10030000, 10040000] 100073 anon

The reason is that the above binary has overlapping elf segments:

  LOAD           0x0000000000000000 0x0000000010000000 0x0000000010000000
                 0x0000000000013a8c 0x0000000000013a8c  R E    10000
  LOAD           0x000000000001fd40 0x000000001002fd40 0x000000001002fd40
                 0x00000000000002c0 0x00000000000005e8  RW     10000
  LOAD           0x0000000000020328 0x0000000010030328 0x0000000010030328
                 0x0000000000000384 0x00000000000094a0  RW     10000

That binary has two RW LOAD segments, the first crosses a page border
into the second

  0x1002fd40 (LOAD2-vaddr) + 0x5e8 (LOAD2-memlen) == 0x10030328 (LOAD3-vaddr)

Handle this situation by enforcing MAP_FIXED when we establish a
temporary brk VMA to handle overlapping segments.  All other mappings
will still use MAP_FIXED_NOREPLACE.

Link: http://lkml.kernel.org/r/20180213100440.GM3443@dhcp22.suse.cz


Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Reviewed-by: Khalid Aziz <khalid.aziz@oracle.com>
Cc: Andrei Vagin <avagin@openvz.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Kees Cook <keescook@chromium.org>
Cc: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Cc: Joel Stanley <joel@jms.id.au>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>