[PATCH 2/3] kasan: record and print the free track

[Walter Wu]

We add new KASAN_RCU_STACK_RECORD configuration option. It will move
free track from slub meta-data (struct kasan_alloc_meta) into freed object.
Because we hope this options doesn't enlarge slub meta-data size.

This option doesn't enlarge struct kasan_alloc_meta size.
- add two call_rcu() call stack into kasan_alloc_meta, size is 8 bytes.
- remove free track from kasan_alloc_meta, size is 8 bytes.

This option is only suitable for generic KASAN, because we move free track
into the freed object, so free track is valid information only when it
exists in quarantine. If the object is in-use state, then the KASAN report
doesn't print call_rcu() free track information.

[1]https://bugzilla.kernel.org/show_bug.cgi?id=198437

Signed-off-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
---
 mm/kasan/common.c | 10 +++++++++-
 mm/kasan/report.c | 24 +++++++++++++++++++++---
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 32d422bdf127..13ec03e225a7 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -321,8 +321,15 @@ void kasan_record_callrcu(void *addr)
 		/* record last call_rcu() call stack */
 		alloc_info->rcu_free_stack[1] = save_stack(GFP_NOWAIT);
 }
-#endif
 
+static void kasan_set_free_info(struct kmem_cache *cache,
+		void *object, u8 tag)
+{
+	/* store free track into freed object */
+	set_track((struct kasan_track *)(object + BYTES_PER_WORD), GFP_NOWAIT);
+}
+
+#else
 static void kasan_set_free_info(struct kmem_cache *cache,
 		void *object, u8 tag)
 {
@@ -339,6 +346,7 @@ static void kasan_set_free_info(struct kmem_cache *cache,
 
 	set_track(&alloc_meta->free_track[idx], GFP_NOWAIT);
 }
+#endif
 
 void kasan_poison_slab(struct page *page)
 {
diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index 7aaccc70b65b..f2b0c6b9dffa 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -175,8 +175,23 @@ static void kasan_print_rcu_free_stack(struct kasan_alloc_meta *alloc_info)
 	print_track(&free_track, "Last call_rcu() call stack", true);
 	pr_err("\n");
 }
-#endif
 
+static struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
+		void *object, u8 tag, const void *addr)
+{
+	u8 *shadow_addr = (u8 *)kasan_mem_to_shadow(addr);
+
+	/*
+	 * Only the freed object can get free track,
+	 * because free track information is stored to freed object.
+	 */
+	if (*shadow_addr == KASAN_KMALLOC_FREE)
+		return (struct kasan_track *)(object + BYTES_PER_WORD);
+	else
+		return NULL;
+}
+
+#else
 static struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
 		void *object, u8 tag, const void *addr)
 {
@@ -196,6 +211,7 @@ static struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
 
 	return &alloc_meta->free_track[i];
 }
+#endif
 
 static void describe_object(struct kmem_cache *cache, void *object,
 				const void *addr, u8 tag)
@@ -208,8 +224,10 @@ static void describe_object(struct kmem_cache *cache, void *object,
 		print_track(&alloc_info->alloc_track, "Allocated", false);
 		pr_err("\n");
 		free_track = kasan_get_free_track(cache, object, tag, addr);
-		print_track(free_track, "Freed", false);
-		pr_err("\n");
+		if (free_track) {
+			print_track(free_track, "Freed", false);
+			pr_err("\n");
+		}
 #ifdef CONFIG_KASAN_RCU_STACK_RECORD
 		kasan_print_rcu_free_stack(alloc_info);
 #endif
-- 
2.18.0