On Thu, Jan 13, 2022 at 6:14 AM Peter Collingbourne <pcc@xxxxxxxxxx> 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. Is it know how exactly this race happens? Why are flags for a newly allocated page being accessed concurrently? > Fix the problem by using a compare-exchange > loop to update the tag. > > Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx> > Link: https://linux-review.googlesource.com/id/I456b24a2b9067d93968d43b4bb3351c0cec63101 > Fixes: 2813b9c02962 ("kasan, mm, arm64: tag non slab memory allocated via pagealloc") > Cc: stable@xxxxxxxxxxxxxxx > --- > include/linux/mm.h | 16 +++++++++++----- > 1 file changed, 11 insertions(+), 5 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index c768a7c81b0b..b544b0a9f537 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -1531,11 +1531,17 @@ 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; > + do { > + old_flags = flags = page->flags; I guess this should be at least READ_ONCE(page->flags) if we care about concurrency. > + flags &= ~(KASAN_TAG_MASK << KASAN_TAG_PGSHIFT); > + flags |= (tag & KASAN_TAG_MASK) << KASAN_TAG_PGSHIFT; > + } while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags)); > } > > static inline void page_kasan_tag_reset(struct page *page) > -- > 2.34.1.575.g55b058a8bb-goog >
