util/cbmem: mmap underflow on low addresses

There is code to adjust the mapping down if a mmap fails
at a physical address. However, if the address is less
than the page size of the system then the physical offset will
underflow. This can actually cause a kernel panic on when
operating on /dev/mem.

The failing condition happens when the requested mapping at 0
fails in the kernel. The fallback path is taken and page size
is subtracted from 0 making a very large offset. The PAT code
in the kernel fails with a BUG_ON in reserve_memtype() checking
start >= end. The kernel needs to be fixed as well, but this
fallback path is wrong as well.

BUG=b:38211793

Change-Id: I32b0c15b2f1aa43fc57656d5d2d5f0e4e90e94ef
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/19679
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philippe.mathieu.daude@gmail.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/cbmem/cbmem.c

@@ -165,7 +165,9 @@ static void *map_memory_size(u64 physical, size_t size, uint8_t abort_on_failure
 
 	v = mmap(NULL, size, PROT_READ, MAP_SHARED, mem_fd, p);
 
-	if (v == MAP_FAILED) {
+	/* Only try growing down when address exceeds page size so that
+	   one doesn't underflow the offset request. */
+	if (v == MAP_FAILED && p >= page) {
 		/* The mapped area may have overrun the upper cbmem boundary when trying to
 		 * align to the page size.  Try growing down instead of up...
 		 */