On Fri, Jan 14, 2022 at 1:58 PM Andrey Konovalov <andreyknvl@xxxxxxxxx> wrote: > > 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? In the report that we received, the race resulted in a crash in kswapd. This may just be a symptom of the problem though. I haven't closely audited all of the callers to page_kasan_tag_set() to check whether they may be operating on already-visible pages, but at least it doesn't appear to be unanticipated that there may be other threads accessing the page flags concurrently with a call to page_kasan_tag_set() (see the calls to smp_wmb() in arch/arm64/kernel/mte.c, arch/arm64/mm/copypage.c and arch/arm64/mm/mteswap.c). > > 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. Makes sense. I copied this code from page_cpupid_xchg_last() in mm/mmzone.c which has the same problem. I'll send a patch to fix that one as well. Peter
