Currently we just dump stack in case of double free bug.
Let's dump all info about the object that we have.

Signed-off-by: Andrey Ryabinin <redacted>
---
 mm/kasan/kasan.c  |  3 +--
 mm/kasan/kasan.h  |  2 ++
 mm/kasan/report.c | 54 ++++++++++++++++++++++++++++++++++++++----------------
 3 files changed, 41 insertions(+), 18 deletions(-)

diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index 92750e3..88af13c 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c

@@ -543,8 +543,7 @@ bool kasan_slab_free(struct kmem_cache *cache, void *object)

        shadow_byte = READ_ONCE(*(s8 *)kasan_mem_to_shadow(object));
        if (shadow_byte < 0 || shadow_byte >= KASAN_SHADOW_SCALE_SIZE) {
-               pr_err("Double free");
-               dump_stack();
+               kasan_report_double_free(cache, object, shadow_byte);
                return true;
        }

diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 9b7b31e..e5c2181 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h

@@ -99,6 +99,8 @@ static inline bool kasan_report_enabled(void)

 void kasan_report(unsigned long addr, size_t size,
                bool is_write, unsigned long ip);
+void kasan_report_double_free(struct kmem_cache *cache, void *object,
+                       s8 shadow);

 #if defined(CONFIG_SLAB) || defined(CONFIG_SLUB)
 void quarantine_put(struct kasan_free_meta *info, struct kmem_cache *cache);

diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index f437398..ee2bdb4 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c

@@ -116,6 +116,26 @@ static inline bool init_task_stack_addr(const void *addr)
                        sizeof(init_thread_union.stack));
 }

+static DEFINE_SPINLOCK(report_lock);
+
+static void kasan_start_report(unsigned long *flags)
+{
+       /*
+        * Make sure we don't end up in loop.
+        */
+       kasan_disable_current();
+       spin_lock_irqsave(&report_lock, *flags);
+       pr_err("==================================================================\n");
+}
+
+static void kasan_end_report(unsigned long *flags)
+{
+       pr_err("==================================================================\n");
+       add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);

+       spin_unlock_irqrestore(&report_lock, *flags);
+       kasan_enable_current();
+}
+
 static void print_track(struct kasan_track *track)
 {
        pr_err("PID = %u\n", track->pid);

@@ -129,8 +149,7 @@ static void print_track(struct kasan_track *track)
        }
 }

-static void kasan_object_err(struct kmem_cache *cache, struct page *page,
-                               void *object, char *unused_reason)
+static void kasan_object_err(struct kmem_cache *cache, void *object)
 {
        struct kasan_alloc_meta *alloc_info = get_alloc_info(cache, object);

@@ -147,6 +166,18 @@ static void kasan_object_err(struct kmem_cache *cache, struct page *page,
        print_track(&alloc_info->free_track);
 }

+void kasan_report_double_free(struct kmem_cache *cache, void *object,
+                       s8 shadow)
+{
+       unsigned long flags;
+
+       kasan_start_report(&flags);
+       pr_err("BUG: Double free or corrupt pointer\n");

+       pr_err("Unexpected shadow byte: 0x%hhX\n", shadow);
+       kasan_object_err(cache, object);
+       kasan_end_report(&flags);
+}
+
 static void print_address_description(struct kasan_access_info *info)
 {
        const void *addr = info->access_addr;

@@ -160,8 +191,7 @@ static void print_address_description(struct kasan_access_info *info)
                        struct kmem_cache *cache = page->slab_cache;
                        object = nearest_obj(cache, page,
                                                (void *)info->access_addr);
-                       kasan_object_err(cache, page, object,
-                                       "kasan: bad access detected");
+                       kasan_object_err(cache, object);
                        return;
                }
                dump_page(page, "kasan: bad access detected");

@@ -226,19 +256,13 @@ static void print_shadow_for_address(const void *addr)
        }
 }

-static DEFINE_SPINLOCK(report_lock);
-
 static void kasan_report_error(struct kasan_access_info *info)
 {
        unsigned long flags;
        const char *bug_type;

-       /*
-        * Make sure we don't end up in loop.
-        */
-       kasan_disable_current();
-       spin_lock_irqsave(&report_lock, flags);
-       pr_err("==================================================================\n");
+       kasan_start_report(&flags);
+
        if (info->access_addr <
                        kasan_shadow_to_mem((void *)KASAN_SHADOW_START)) {
                if ((unsigned long)info->access_addr < PAGE_SIZE)

@@ -259,10 +283,8 @@ static void kasan_report_error(struct kasan_access_info *info)
                print_address_description(info);
                print_shadow_for_address(info->first_bad_addr);
        }
-       pr_err("==================================================================\n");
-       add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
-       spin_unlock_irqrestore(&report_lock, flags);
-       kasan_enable_current();
+
+       kasan_end_report(&flags);
 }

 void kasan_report(unsigned long addr, size_t size,
--

2.7.3