mm: kasan: Ensure the tags are visible before the tag in page->flags

__kasan_unpoison_pages() colours the memory with a random tag and stores
it in page->flags in order to re-create the tagged pointer via
page_to_virt() later. When the tag from the page->flags is read, ensure
that the in-memory tags are already visible by re-ordering the
page_kasan_tag_set() after kasan_unpoison(). The former already has
barriers in place through try_cmpxchg(). On the reader side, the order
is ensured by the address dependency between page->flags and the memory
access.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20220610152141.2148929-2-catalin.marinas@arm.comSigned-off-by: Will Deacon <will@kernel.org>

mm/kasan/common.c

@@ -108,9 +108,10 @@ void __kasan_unpoison_pages(struct page *page, unsigned int order, bool init)
 		return;
 
 	tag = kasan_random_tag();
+	kasan_unpoison(set_tag(page_address(page), tag),
+		       PAGE_SIZE << order, init);
 	for (i = 0; i < (1 << order); i++)
 		page_kasan_tag_set(page + i, tag);
-	kasan_unpoison(page_address(page), PAGE_SIZE << order, init);
 }
 
 void __kasan_poison_pages(struct page *page, unsigned int order, bool init)