[PATCH mm v4 18/19] kasan, mm: allow cache merging with no metadata

[Andrey Konovalov]

The reason cache merging is disabled with KASAN is because KASAN puts its
metadata right after the allocated object. When the merged caches have
slightly different sizes, the metadata ends up in different places, which
KASAN doesn't support.

It might be possible to adjust the metadata allocation algorithm and make
it friendly to the cache merging code. Instead this change takes a simpler
approach and allows merging caches when no metadata is present. Which is
the case for hardware tag-based KASAN with kasan.mode=prod.

Co-developed-by: Vincenzo Frascino <Vincenzo.Frascino at arm.com>
Signed-off-by: Vincenzo Frascino <Vincenzo.Frascino at arm.com>
Signed-off-by: Andrey Konovalov <andreyknvl at google.com>
Reviewed-by: Dmitry Vyukov <dvyukov at google.com>
Reviewed-by: Marco Elver <elver at google.com>
Link: https://linux-review.googlesource.com/id/Ia114847dfb2244f297d2cb82d592bf6a07455dba
---
 include/linux/kasan.h | 21 +++++++++++++++++++--
 mm/kasan/common.c     | 11 +++++++++++
 mm/slab_common.c      |  3 ++-
 3 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 2610438120ce..83860aa4e89c 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -81,17 +81,30 @@ struct kasan_cache {
 };
 
 #ifdef CONFIG_KASAN_HW_TAGS
+
 DECLARE_STATIC_KEY_FALSE(kasan_flag_enabled);
+
 static __always_inline bool kasan_enabled(void)
 {
 	return static_branch_likely(&kasan_flag_enabled);
 }
-#else
+
+#else /* CONFIG_KASAN_HW_TAGS */
+
 static inline bool kasan_enabled(void)
 {
 	return true;
 }
-#endif
+
+#endif /* CONFIG_KASAN_HW_TAGS */
+
+slab_flags_t __kasan_never_merge(void);
+static __always_inline slab_flags_t kasan_never_merge(void)
+{
+	if (kasan_enabled())
+		return __kasan_never_merge();
+	return 0;
+}
 
 void __kasan_unpoison_range(const void *addr, size_t size);
 static __always_inline void kasan_unpoison_range(const void *addr, size_t size)
@@ -238,6 +251,10 @@ static inline bool kasan_enabled(void)
 {
 	return false;
 }
+static inline slab_flags_t kasan_never_merge(void)
+{
+	return 0;
+}
 static inline void kasan_unpoison_range(const void *address, size_t size) {}
 static inline void kasan_alloc_pages(struct page *page, unsigned int order) {}
 static inline void kasan_free_pages(struct page *page, unsigned int order) {}
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 249ccba1ecf5..da79f340f3a6 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -87,6 +87,17 @@ asmlinkage void kasan_unpoison_task_stack_below(const void *watermark)
 }
 #endif /* CONFIG_KASAN_STACK */
 
+/*
+ * Only allow cache merging when stack collection is disabled and no metadata
+ * is present.
+ */
+slab_flags_t __kasan_never_merge(void)
+{
+	if (kasan_stack_collection_enabled())
+		return SLAB_KASAN;
+	return 0;
+}
+
 void __kasan_alloc_pages(struct page *page, unsigned int order)
 {
 	u8 tag;
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 0b5ae1819a8b..075b23ce94ec 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -19,6 +19,7 @@
 #include <linux/seq_file.h>
 #include <linux/proc_fs.h>
 #include <linux/debugfs.h>
+#include <linux/kasan.h>
 #include <asm/cacheflush.h>
 #include <asm/tlbflush.h>
 #include <asm/page.h>
@@ -54,7 +55,7 @@ static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
  */
 #define SLAB_NEVER_MERGE (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER | \
 		SLAB_TRACE | SLAB_TYPESAFE_BY_RCU | SLAB_NOLEAKTRACE | \
-		SLAB_FAILSLAB | SLAB_KASAN)
+		SLAB_FAILSLAB | kasan_never_merge())
 
 #define SLAB_MERGE_SAME (SLAB_RECLAIM_ACCOUNT | SLAB_CACHE_DMA | \
 			 SLAB_CACHE_DMA32 | SLAB_ACCOUNT)
-- 
2.29.2.454.gaff20da3a2-goog