On Mon, Jan 24, 2022 at 6:22 PM Andrey Konovalov andreyknvl@gmail.com wrote:

On Thu, Jan 20, 2022 at 3:02 AM Peter Collingbourne pcc@google.com wrote:

...

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.

Signed-off-by: Peter Collingbourne pcc@google.com Link: https://linux-review.googlesource.com/id/I456b24a2b9067d93968d43b4bb3351c0ce... Fixes: 2813b9c02962 ("kasan, mm, arm64: tag non slab memory allocated via pagealloc") Cc: stable@vger.kernel.org

---

v3:

• use try_cmpxchg() as suggested by Peter Zijlstra on another patch

v2:

• use READ_ONCE()

include/linux/mm.h | 17 ++++++++++++----- 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h index c768a7c81b0b..87473fe52c3f 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1531,11 +1531,18 @@ 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)

2.34.1.703.g22d0c6ccf7-goog

Reviewed-by: Andrey Konovalov andreyknvl@gmail.com

FWIW, try_cmpxchg() doesn't seem to be doing annotated atomic accesses when accessing old_flags, so using READ_ONCE() in page_kasan_tag_set() seems pointless after all.

Ah, nevermind. For a second I thought that try_cmpxchg() is actually accessing page->flags through its second argument.