s390/kasan: handle DCSS mapping in memory holes

When physical memory is defined under z/VM using DEF STOR CONFIG, there
may be memory holes that are not hotpluggable memory. In such cases,
DCSS mapping could be placed in one of these memory holes. Subsequently,
attempting memory access to such DCSS mapping would result in a kasan
failure because there is no shadow memory mapping for it.

To maintain consistency with cases where DCSS mapping is positioned after
the kernel identity mapping, which is then covered by kasan zero shadow
mapping, handle the scenario above by populating zero shadow mapping
for memory holes where DCSS mapping could potentially be placed.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/boot/vmem.c

@@ -57,6 +57,7 @@ static void kasan_populate_shadow(void)
 	pmd_t pmd_z = __pmd(__pa(kasan_early_shadow_pte) | _SEGMENT_ENTRY);
 	pud_t pud_z = __pud(__pa(kasan_early_shadow_pmd) | _REGION3_ENTRY);
 	p4d_t p4d_z = __p4d(__pa(kasan_early_shadow_pud) | _REGION2_ENTRY);
+	unsigned long memgap_start = 0;
 	unsigned long untracked_end;
 	unsigned long start, end;
 	int i;
@@ -101,8 +102,12 @@ static void kasan_populate_shadow(void)
 	 * +- shadow end ----+---------+- shadow end ---+
 	 */
 
-	for_each_physmem_usable_range(i, &start, &end)
+	for_each_physmem_usable_range(i, &start, &end) {
 		kasan_populate(start, end, POPULATE_KASAN_MAP_SHADOW);
+		if (memgap_start && physmem_info.info_source == MEM_DETECT_DIAG260)
+			kasan_populate(memgap_start, start, POPULATE_KASAN_ZERO_SHADOW);
+		memgap_start = end;
+	}
 	if (IS_ENABLED(CONFIG_KASAN_VMALLOC)) {
 		untracked_end = VMALLOC_START;
 		/* shallowly populate kasan shadow for vmalloc and modules */