ELF: fix overflow in total mapping size calculation

ELF: fix overflow in total mapping size calculation

Kernel assumes that ELF program headers are ordered by mapping address,
but doesn't enforce it.  It is possible to make mapping size extremely
huge by simply shuffling first and last PT_LOAD segments.

As long as PT_LOAD segments do not overlap, it is silly to require sorting
by v_addr anyway because mmap() doesn't care.

Don't assume PT_LOAD segments are sorted and calculate min and max
addresses correctly.

Link: https://lkml.kernel.org/r/YVmd7D0M6G/DcP4O@localhost.localdomain
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>