elf: fix NT_FILE integer overflow

If vm.max_map_count bumped above 2^26 (67+ mil) and system has enough RAM
to allocate all the VMAs (~12.8 GB on Fedora 27 with 200-byte VMAs), then
it should be possible to overflow 32-bit "size", pass paranoia check,
allocate very little vmalloc space and oops while writing into vmalloc
guard page...

But I didn't test this, only coredump of regular process.

Link: http://lkml.kernel.org/r/20180112203427.GA9109@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_elf.c

@@ -1599,6 +1599,8 @@ static int fill_files_note(struct memelfnote *note)
 
 	/* *Estimated* file count and total data size needed */
 	count = current->mm->map_count;
+	if (count > UINT_MAX / 64)
+		return -EINVAL;
 	size = count * 64;
 
 	names_ofs = (2 + 3 * count) * sizeof(data[0]);