[PATCH] mm/page_alloc: silence a KASAN false positive

[Qian Cai]

kernel_init_free_pages() will use memset() on s390 to clear all pages
from kmalloc_order() which will override KASAN redzones because a
redzone was setup from the end of the allocation size to the end of the
last page. Silence it by not reporting it there. An example of the
report is,

 BUG: KASAN: slab-out-of-bounds in __free_pages_ok
 Write of size 4096 at addr 000000014beaa000
 Call Trace:
 show_stack+0x152/0x210
 dump_stack+0x1f8/0x248
 print_address_description.isra.13+0x5e/0x4d0
 kasan_report+0x130/0x178
 check_memory_region+0x190/0x218
 memset+0x34/0x60
 __free_pages_ok+0x894/0x12f0
 kfree+0x4f2/0x5e0
 unpack_to_rootfs+0x60e/0x650
 populate_rootfs+0x56/0x358
 do_one_initcall+0x1f4/0xa20
 kernel_init_freeable+0x758/0x7e8
 kernel_init+0x1c/0x170
 ret_from_fork+0x24/0x28
 Memory state around the buggy address:
 000000014bea9f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 000000014bea9f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>000000014beaa000: 03 fe fe fe fe fe fe fe fe fe fe fe fe fe fe fe
                    ^
 000000014beaa080: fe fe fe fe fe fe fe fe fe fe fe fe fe fe fe fe
 000000014beaa100: fe fe fe fe fe fe fe fe fe fe fe fe fe fe

Fixes: 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options")
Signed-off-by: Qian Cai <cai@xxxxxx>
---
 mm/page_alloc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 727751219003..9954973f89a3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1164,8 +1164,11 @@ static void kernel_init_free_pages(struct page *page, int numpages)
 {
 	int i;
 
+	/* s390's use of memset() could override KASAN redzones. */
+	kasan_disable_current();
 	for (i = 0; i < numpages; i++)
 		clear_highpage(page + i);
+	kasan_enable_current();
 }
 
 static __always_inline bool free_pages_prepare(struct page *page,
-- 
2.21.0 (Apple Git-122.2)