2022-01-13 03:14:47

by Peter Collingbourne

[permalink] [raw]
Subject: [PATCH] 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]
---
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;
+ 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



2022-01-13 03:32:47

by Matthew Wilcox

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

On Wed, Jan 12, 2022 at 07:14:34PM -0800, Peter Collingbourne 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.

I really dislike it that kasan has taken some page flags for its use.
I would much prefer it if kasan used some page_ext_flags. It's somewhat
slower to access them, and they take up a bit of extra space (unless
you already have CONFIG_PAGE_EXTENSION enabled). But page flags are a
really scarce resource and kasan has taken 9.

2022-01-14 23:09:02

by Andrey Konovalov

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

On Thu, Jan 13, 2022 at 6:14 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.

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 <[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]
> ---
> 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
>