s390/boot: Do not assume the decompressor range is reserved

[ Upstream commit b798b68 ]

When allocating a random memory range for .amode31 sections
the minimal randomization address is 0. That does not lead
to a possible overlap with the decompressor image (which also
starts from 0) since by that time the image range is already
reserved.

Do not assume the decompressor range is reserved and always
provide the minimal randomization address for .amode31
sections beyond the decompressor. That is a prerequisite
for moving the lowcore memory address from NULL elsewhere.

Signed-off-by: Alexander Gordeev <[email protected]>
Signed-off-by: Sven Schnelle <[email protected]>
Signed-off-by: Vasily Gorbik <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>

arch/s390/boot/startup.c

@@ -476,8 +476,12 @@ void startup_kernel(void)
 	 * before the kernel started. Therefore, in case the two sections
 	 * overlap there is no risk of corrupting any data.
 	 */
-	if (kaslr_enabled())
-		amode31_lma = randomize_within_range(vmlinux.amode31_size, PAGE_SIZE, 0, SZ_2G);
+	if (kaslr_enabled()) {
+		unsigned long amode31_min;
+
+		amode31_min = (unsigned long)_decompressor_end;
+		amode31_lma = randomize_within_range(vmlinux.amode31_size, PAGE_SIZE, amode31_min, SZ_2G);
+	}
 	if (!amode31_lma)
 		amode31_lma = text_lma - vmlinux.amode31_size;
 	physmem_reserve(RR_AMODE31, amode31_lma, vmlinux.amode31_size);