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mm: use compare-exchange operation to set KASAN page tag

PATCH In review Peter Collingbourne 2 participants 3 messages Jan 20, 2022 – Jan 24, 2022

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Stable branch Memory management MM core
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Peter Collingbourne

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#0 lore raw

PATCH v3 1/1 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(-)
include/linux/mm.h +12 -5
index c768a7c81b0b..87473fe52c3f 100644
@@ -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)
Signature · 1 line
2.34.1.703.g22d0c6ccf7-goog
Reviewed-by on v3 1/1
#1 lore raw
Quoted 51 lines · Peter Collingbourne

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(-)
include/linux/mm.h +12 -6
index c768a7c81b0b..87473fe52c3f 100644
@@ -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)
--
Quoted 2 lines

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.

#2 lore raw
Quoted 58 lines · Andrey Konovalov
Quoted 51 lines · Peter Collingbourne

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(-)
include/linux/mm.h +12 -6
index c768a7c81b0b..87473fe52c3f 100644
@@ -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)
--
Quoted 2 lines

2.34.1.703.g22d0c6ccf7-goog

Reviewed-by Andrey Konovalov <[email protected]>
Quoted 3 lines

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.