s390/kaslr: randomize amode31 base address

When the KASLR is enabled, randomize the base address of the amode31 image
within the first 2 GB, similar to the approach taken for the vmlinux
image. This makes it harder to predict the location of amode31 data
and code.

Reviewed-by: Alexander Gordeev <[e-mail]>
Signed-off-by: Vasily Gorbik <[e-mail]>

arch/s390/boot/startup.c

···
 {
 	unsigned long max_physmem_end;
 	unsigned long vmlinux_lma = 0;
-	unsigned long amode31_lma;
+	unsigned long amode31_lma = 0;
 	unsigned long asce_limit;
 	unsigned long safe_addr;
 	void *img;
···
 
 	/* vmlinux decompression is done, shrink reserved low memory */
 	physmem_reserve(RR_DECOMPRESSOR, 0, (unsigned long)_decompressor_end);
-	amode31_lma = vmlinux.default_lma - vmlinux.amode31_size;
+	if (kaslr_enabled())
+		amode31_lma = randomize_within_range(vmlinux.amode31_size, PAGE_SIZE, 0, SZ_2G);
+	amode31_lma = amode31_lma ?: vmlinux.default_lma - vmlinux.amode31_size;
 	physmem_reserve(RR_AMODE31, amode31_lma, vmlinux.amode31_size);
 
 	/*