Merge: Fix kdump problem on s390x

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9/-/merge_requests/7140

JIRA: https://issues.redhat.com/browse/RHEL-103929

A recent backport introduce a regression that causes the reboot after a kdump to fail on s390x. We need these patches here to fix this issue.

Signed-off-by: Thomas Huth <thuth@redhat.com>

Resolves: RHEL-103929

Approved-by: David Hildenbrand <david@redhat.com>
Approved-by: Steve Best <sbest@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: Augusto Caringi <acaringi@redhat.com>

Author: caringi

arch/s390/Kconfig

@@ -623,6 +623,19 @@ config RANDOMIZE_BASE
 	  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

@@ -336,7 +336,8 @@ static unsigned long setup_kernel_memory_layout(unsigned long kernel_size)
 	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;
 }

arch/s390/kernel/setup.c

@@ -735,7 +735,23 @@ static void __init memblock_add_physmem_info(void)
 }
 
 /*
- * Reserve memory used for lowcore/command line/kernel image.
+ * Reserve memory used for lowcore.
+ */
+static void __init reserve_lowcore(void)
+{
+	void *lowcore_start = get_lowcore();
+	void *lowcore_end = lowcore_start + sizeof(struct lowcore);
+	void *start, *end;
+
+	if ((void *)__identity_base < lowcore_end) {
+		start = max(lowcore_start, (void *)__identity_base);
+		end = min(lowcore_end, (void *)(__identity_base + ident_map_size));
+		memblock_reserve(__pa(start), __pa(end));
+	}
+}
+
+/*
+ * Reserve memory used for absolute lowcore/command line/kernel image.
  */
 static void __init reserve_kernel(void)
 {
@@ -923,6 +939,7 @@ void __init setup_arch(char **cmdline_p)
 
 	/* Do some memory reservations *before* memory is added to memblock */
 	reserve_pgtables();
+	reserve_lowcore();
 	reserve_kernel();
 	reserve_initrd();
 	reserve_certificate_list();

redhat/configs/rhel/generic/s390x/CONFIG_RANDOMIZE_IDENTITY_BASE

@@ -0,0 +1 @@
+# CONFIG_RANDOMIZE_IDENTITY_BASE is not set