s390/mm: Pin identity mapping base to zero

SIE instruction performs faster when the virtual address of
SIE block matches the physical one. Pin the identity mapping
base to zero for the benefit of SIE and other instructions
that have similar performance impact. Still, randomize the
base when DEBUG_VM kernel configuration option is enabled.

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

arch/s390/Kconfig

···
 	  as a security feature that deters exploit attempts relying on
 	  knowledge of the location of kernel internals.
 
+config RANDOMIZE_IDENTITY_BASE
+	bool "Randomize the address of the identity mapping base"
+	depends on RANDOMIZE_BASE
+	default DEBUG_VM
+	help
+	  The identity mapping base address is pinned to zero by default.
+	  Allow randomization of that base to expose otherwise missed
+	  notion of physical and virtual addresses of data structures.
+	  That does not have any impact on the base address at which the
+	  kernel image is loaded.
+
+	  If unsure, say N
+
 config KERNEL_IMAGE_BASE
 	hex "Kernel image base address"
 	range 0x100000 0x1FFFFFE0000000 if !KASAN

arch/s390/boot/startup.c

···
 	BUILD_BUG_ON(MAX_DCSS_ADDR > (1UL << MAX_PHYSMEM_BITS));
 	max_mappable = max(ident_map_size, MAX_DCSS_ADDR);
 	max_mappable = min(max_mappable, vmemmap_start);
-	__identity_base = round_down(vmemmap_start - max_mappable, rte_size);
+	if (IS_ENABLED(CONFIG_RANDOMIZE_IDENTITY_BASE))
+		__identity_base = round_down(vmemmap_start - max_mappable, rte_size);
 
 	return asce_limit;
 }