[PATCH] kasan: separate double free case from invalid free

[Kuan-Ying Lee]

Currently, KASAN describes all invalid-free/double-free bugs as
"double-free or invalid-free". This is ambiguous.

KASAN should report "double-free" when a double-free is a more
likely cause (the address points to the start of an object) and
report "invalid-free" otherwise [1].

[1] https://bugzilla.kernel.org/show_bug.cgi?id=212193

Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@xxxxxxxxxxxx>
---
 mm/kasan/common.c |  8 ++++----
 mm/kasan/kasan.h  |  3 ++-
 mm/kasan/report.c | 12 ++++++++----
 3 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index c40c0e7b3b5f..707c3a527fcb 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -343,7 +343,7 @@ static inline bool ____kasan_slab_free(struct kmem_cache *cache, void *object,
 
 	if (unlikely(nearest_obj(cache, virt_to_slab(object), object) !=
 	    object)) {
-		kasan_report_invalid_free(tagged_object, ip);
+		kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_INVALID_FREE);
 		return true;
 	}
 
@@ -352,7 +352,7 @@ static inline bool ____kasan_slab_free(struct kmem_cache *cache, void *object,
 		return false;
 
 	if (!kasan_byte_accessible(tagged_object)) {
-		kasan_report_invalid_free(tagged_object, ip);
+		kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_DOUBLE_FREE);
 		return true;
 	}
 
@@ -377,12 +377,12 @@ bool __kasan_slab_free(struct kmem_cache *cache, void *object,
 static inline bool ____kasan_kfree_large(void *ptr, unsigned long ip)
 {
 	if (ptr != page_address(virt_to_head_page(ptr))) {
-		kasan_report_invalid_free(ptr, ip);
+		kasan_report_invalid_free(ptr, ip, KASAN_REPORT_INVALID_FREE);
 		return true;
 	}
 
 	if (!kasan_byte_accessible(ptr)) {
-		kasan_report_invalid_free(ptr, ip);
+		kasan_report_invalid_free(ptr, ip, KASAN_REPORT_DOUBLE_FREE);
 		return true;
 	}
 
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 610d60d6e5b8..01c03e45acd4 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -125,6 +125,7 @@ static inline bool kasan_sync_fault_possible(void)
 enum kasan_report_type {
 	KASAN_REPORT_ACCESS,
 	KASAN_REPORT_INVALID_FREE,
+	KASAN_REPORT_DOUBLE_FREE,
 };
 
 struct kasan_report_info {
@@ -277,7 +278,7 @@ static inline void kasan_print_address_stack_frame(const void *addr) { }
 
 bool kasan_report(unsigned long addr, size_t size,
 		bool is_write, unsigned long ip);
-void kasan_report_invalid_free(void *object, unsigned long ip);
+void kasan_report_invalid_free(void *object, unsigned long ip, enum kasan_report_type type);
 
 struct page *kasan_addr_to_page(const void *addr);
 struct slab *kasan_addr_to_slab(const void *addr);
diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index b341a191651d..fe3f606b3a98 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -176,8 +176,12 @@ static void end_report(unsigned long *flags, void *addr)
 static void print_error_description(struct kasan_report_info *info)
 {
 	if (info->type == KASAN_REPORT_INVALID_FREE) {
-		pr_err("BUG: KASAN: double-free or invalid-free in %pS\n",
-		       (void *)info->ip);
+		pr_err("BUG: KASAN: invalid-free in %pS\n", (void *)info->ip);
+		return;
+	}
+
+	if (info->type == KASAN_REPORT_DOUBLE_FREE) {
+		pr_err("BUG: KASAN: double-free in %pS\n", (void *)info->ip);
 		return;
 	}
 
@@ -433,7 +437,7 @@ static void print_report(struct kasan_report_info *info)
 	}
 }
 
-void kasan_report_invalid_free(void *ptr, unsigned long ip)
+void kasan_report_invalid_free(void *ptr, unsigned long ip, enum kasan_report_type type)
 {
 	unsigned long flags;
 	struct kasan_report_info info;
@@ -448,7 +452,7 @@ void kasan_report_invalid_free(void *ptr, unsigned long ip)
 
 	start_report(&flags, true);
 
-	info.type = KASAN_REPORT_INVALID_FREE;
+	info.type = type;
 	info.access_addr = ptr;
 	info.first_bad_addr = kasan_reset_tag(ptr);
 	info.access_size = 0;
-- 
2.18.0