s390/kasan: fix insecure W+X mapping warning

Since commit 3b5c3f000c2e ("s390/kasan: move shadow mapping
to decompressor") the decompressor establishes mappings for
the shadow memory and sets initial protection attributes to
RWX. The decompressed kernel resets protection to RW+NX
later on.

In case a shadow memory range is not aligned on page boundary
(e.g. as result of mem= kernel command line parameter use),
the "Checked W+X mappings: FAILED, 1 W+X pages found" warning
hits.
Reported-by: Vasily Gorbik <gor@linux.ibm.com>
Fixes: 557b1970 ("s390/kasan: move shadow mapping to decompressor")
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/mm/vmem.c

@@ -667,7 +667,15 @@ static void __init memblock_region_swap(void *a, void *b, int size)
 
 #ifdef CONFIG_KASAN
 #define __sha(x)	((unsigned long)kasan_mem_to_shadow((void *)x))
+
+static inline int set_memory_kasan(unsigned long start, unsigned long end)
+{
+	start = PAGE_ALIGN_DOWN(__sha(start));
+	end = PAGE_ALIGN(__sha(end));
+	return set_memory_rwnx(start, (end - start) >> PAGE_SHIFT);
+}
 #endif
+
 /*
  * map whole physical memory to virtual memory (identity mapping)
  * we reserve enough space in the vmalloc area for vmemmap to hotplug
@@ -737,10 +745,8 @@ void __init vmem_map_init(void)
 	}
 
 #ifdef CONFIG_KASAN
-	for_each_mem_range(i, &base, &end) {
-		set_memory_rwnx(__sha(base),
-				(__sha(end) - __sha(base)) >> PAGE_SHIFT);
-	}
+	for_each_mem_range(i, &base, &end)
+		set_memory_kasan(base, end);
 #endif
 	set_memory_rox((unsigned long)_stext,
 		       (unsigned long)(_etext - _stext) >> PAGE_SHIFT);