2022-01-21 20:56:08

by Peter Collingbourne

Subject: [PATCH v3] mm: 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.

Signed-off-by: Peter Collingbourne <[email protected]>
Link: https://linux-review.googlesource.com/id/I456b24a2b9067d93968d43b4bb3351c0cec63101
Fixes: 2813b9c02962 ("kasan, mm, arm64: tag non slab memory allocated via pagealloc")
Cc: [email protected]
---
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

2022-01-24 19:41:13

by Andrey Konovalov

Subject: Re: [PATCH v3] mm: use compare-exchange operation to set KASAN page tag

On Thu, Jan 20, 2022 at 3:02 AM Peter Collingbourne <[email protected]> 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 <[email protected]>
> Link: https://linux-review.googlesource.com/id/I456b24a2b9067d93968d43b4bb3351c0cec63101
> Fixes: 2813b9c02962 ("kasan, mm, arm64: tag non slab memory allocated via pagealloc")
> Cc: [email protected]
> ---
> 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 <[email protected]>

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.

2022-01-24 19:41:52

by Andrey Konovalov

Subject: Re: [PATCH v3] mm: use compare-exchange operation to set KASAN page tag

On Mon, Jan 24, 2022 at 6:22 PM Andrey Konovalov <[email protected]> wrote:
>
> On Thu, Jan 20, 2022 at 3:02 AM Peter Collingbourne <[email protected]> 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 <[email protected]>
> > Link: https://linux-review.googlesource.com/id/I456b24a2b9067d93968d43b4bb3351c0cec63101
> > Fixes: 2813b9c02962 ("kasan, mm, arm64: tag non slab memory allocated via pagealloc")
> > Cc: [email protected]
> > ---
> > 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 <[email protected]>
>
> 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.