kasan: fix per-page tags for non-page_alloc pages

To allow performing tag checks on page_alloc addresses obtained via
page_address(), tag-based KASAN modes store tags for page_alloc
allocations in page->flags.

Currently, the default tag value stored in page->flags is 0x00.
Therefore, page_address() returns a 0x00ffff...  address for pages that
were not allocated via page_alloc.

This might cause problems.  A particular case we encountered is a
conflict with KFENCE.  If a KFENCE-allocated slab object is being freed
via kfree(page_address(page) + offset), the address passed to kfree()
will get tagged with 0x00 (as slab pages keep the default per-page
tags).  This leads to is_kfence_address() check failing, and a KFENCE
object ending up in normal slab freelist, which causes memory
corruptions.

This patch changes the way KASAN stores tag in page-flags: they are now
stored xor'ed with 0xff.  This way, KASAN doesn't need to initialize
per-page flags for every created page, which might be slow.

With this change, page_address() returns natively-tagged (with 0xff)
pointers for pages that didn't have tags set explicitly.

This patch fixes the encountered conflict with KFENCE and prevents more
similar issues that can occur in the future.

Link: https://lkml.kernel.org/r/1a41abb11c51b264511d9e71c303bb16d5cb367b.1615475452.git.andreyknvl@google.com
Fixes: 2813b9c0 ("kasan, mm, arm64: tag non slab memory allocated via pagealloc")
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Branislav Rankov <Branislav.Rankov@arm.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -1461,16 +1461,28 @@ static inline bool cpupid_match_pid(struct task_struct *task, int cpupid)
 
 #if defined(CONFIG_KASAN_SW_TAGS) || defined(CONFIG_KASAN_HW_TAGS)
 
+/*
+ * KASAN per-page tags are stored xor'ed with 0xff. This allows to avoid
+ * setting tags for all pages to native kernel tag value 0xff, as the default
+ * value 0x00 maps to 0xff.
+ */
+
 static inline u8 page_kasan_tag(const struct page *page)
 {
-	if (kasan_enabled())
-		return (page->flags >> KASAN_TAG_PGSHIFT) & KASAN_TAG_MASK;
-	return 0xff;
+	u8 tag = 0xff;
+
+	if (kasan_enabled()) {
+		tag = (page->flags >> KASAN_TAG_PGSHIFT) & KASAN_TAG_MASK;
+		tag ^= 0xff;
+	}
+
+	return tag;
 }
 
 static inline void page_kasan_tag_set(struct page *page, u8 tag)
 {
 	if (kasan_enabled()) {
+		tag ^= 0xff;
 		page->flags &= ~(KASAN_TAG_MASK << KASAN_TAG_PGSHIFT);
 		page->flags |= (tag & KASAN_TAG_MASK) << KASAN_TAG_PGSHIFT;
 	}