mm, kasan: use compare-exchange operation to set KASAN page tag

It has been reported that the tag setting operation on newly-allocated
pages can cause the page flags to be corrupted when performed
concurrently with other flag updates as a result of the use of
non-atomic operations.

Fix the problem by using a compare-exchange loop to update the tag.

Link: https://lkml.kernel.org/r/20220120020148.1632253-1-pcc@google.com
Link: https://linux-review.googlesource.com/id/I456b24a2b9067d93968d43b4bb3351c0cec63101
Fixes: 2813b9c02962 ("kasan, mm, arm64: tag non slab memory allocated via pagealloc")
Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index e1a84b1..213cc56 100644 (file)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1506,11 +1506,18 @@ static inline u8 page_kasan_tag(const struct page *page)
 
 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;
-       }
+       unsigned long old_flags, flags;
+
+       if (!kasan_enabled())
+               return;
+
+       tag ^= 0xff;
+       old_flags = READ_ONCE(page->flags);
+       do {
+               flags = old_flags;
+               flags &= ~(KASAN_TAG_MASK << KASAN_TAG_PGSHIFT);
+               flags |= (tag & KASAN_TAG_MASK) << KASAN_TAG_PGSHIFT;
+       } while (unlikely(!try_cmpxchg(&page->flags, &old_flags, flags)));
 }
 
 static inline void page_kasan_tag_reset(struct page *page)