[PATCH v3 1/2] mm, kasan: improve double-free detection

From: Kuthonuzo Luruo
Date: Tue May 24 2016 - 14:40:38 EST

Currently, KASAN may fail to detect concurrent deallocations of the same
object due to a race in kasan_slab_free(). This patch makes double-free
detection more reliable by serializing access to KASAN object metadata.
New functions kasan_meta_lock() and kasan_meta_unlock() are provided to
lock/unlock per-object metadata. Double-free errors are now reported via
kasan_report().

Per-object lock concept from suggestion/observations by Dmitry Vyukov.

Testing:
- Tested with a modified version of the 'slab_test' microbenchmark where
allocs occur on CPU 0; then all other CPUs concurrently attempt to free
the same object.
- Tested with new double-free tests for 'test_kasan' in accompanying patch.

Signed-off-by: Kuthonuzo Luruo <kuthonuzo.luruo@xxxxxxx>
---

Changes in v3:
- simplified kasan_meta_lock()/unlock() to use generic bit spinlock apis;
kasan_alloc_meta structure modified accordingly.
- introduced a 'safety valve' for kasan_meta_lock() to prevent a kfree from
getting stuck when a prior out-of-bounds write clobbers the object
header.
- removed potentially unsafe __builtin_return_address(1) from
kasan_report() call per review comment from Yury Norov; callee now passed
into kasan_slab_free().

---
include/linux/kasan.h | 7 +++-
mm/kasan/kasan.c | 88 ++++++++++++++++++++++++++++++++++---------------
mm/kasan/kasan.h | 44 +++++++++++++++++++++++-
mm/kasan/quarantine.c | 2 +
mm/kasan/report.c | 28 ++++++++++++++--
mm/slab.c | 3 +-
mm/slub.c | 2 +-
7 files changed, 138 insertions(+), 36 deletions(-)

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 611927f..3db974b 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -53,6 +53,7 @@ void kasan_cache_create(struct kmem_cache *cache, size_t *size,
void kasan_cache_shrink(struct kmem_cache *cache);
void kasan_cache_destroy(struct kmem_cache *cache);

+void kasan_init_object(struct kmem_cache *cache, void *object);
void kasan_poison_slab(struct page *page);
void kasan_unpoison_object_data(struct kmem_cache *cache, void *object);
void kasan_poison_object_data(struct kmem_cache *cache, void *object);
@@ -65,7 +66,7 @@ void kasan_kmalloc(struct kmem_cache *s, const void *object, size_t size,
void kasan_krealloc(const void *object, size_t new_size, gfp_t flags);

void kasan_slab_alloc(struct kmem_cache *s, void *object, gfp_t flags);
-bool kasan_slab_free(struct kmem_cache *s, void *object);
+bool kasan_slab_free(struct kmem_cache *s, void *object, unsigned long caller);
void kasan_poison_slab_free(struct kmem_cache *s, void *object);

struct kasan_cache {
@@ -94,6 +95,7 @@ static inline void kasan_cache_create(struct kmem_cache *cache,
static inline void kasan_cache_shrink(struct kmem_cache *cache) {}
static inline void kasan_cache_destroy(struct kmem_cache *cache) {}

+static inline void kasan_init_object(struct kmem_cache *s, void *object) {}
static inline void kasan_poison_slab(struct page *page) {}
static inline void kasan_unpoison_object_data(struct kmem_cache *cache,
void *object) {}
@@ -110,7 +112,8 @@ static inline void kasan_krealloc(const void *object, size_t new_size,

static inline void kasan_slab_alloc(struct kmem_cache *s, void *object,
gfp_t flags) {}
-static inline bool kasan_slab_free(struct kmem_cache *s, void *object)
+static inline bool kasan_slab_free(struct kmem_cache *s, void *object,
+ unsigned long caller)
{
return false;
}
diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index 18b6a2b..ab82e24 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c
@@ -402,6 +402,35 @@ void kasan_cache_create(struct kmem_cache *cache, size_t *size,
cache->object_size +
optimal_redzone(cache->object_size)));
}
+
+void kasan_init_object(struct kmem_cache *cache, void *object)
+{
+ if (cache->flags & SLAB_KASAN) {
+ struct kasan_alloc_meta *alloc_info;
+
+ alloc_info = get_alloc_info(cache, object);
+ __memset(alloc_info, 0, sizeof(*alloc_info));
+ }
+}
+
+/* flags shadow for object header if it has been overwritten. */
+void kasan_mark_bad_meta(struct kasan_alloc_meta *alloc_info,
+ struct kasan_access_info *info)
+{
+ u8 *datap = (u8 *)&alloc_info->data;
+
+ if ((((u8 *)info->access_addr + info->access_size) > datap) &&
+ ((u8 *)info->first_bad_addr <= datap) &&
+ info->is_write)
+ kasan_poison_shadow((void *)datap, KASAN_SHADOW_SCALE_SIZE,
+ KASAN_KMALLOC_BAD_META);
+}
+
+static void kasan_unmark_bad_meta(struct kasan_alloc_meta *alloc_info)
+{
+ kasan_poison_shadow((void *)&alloc_info->data, KASAN_SHADOW_SCALE_SIZE,
+ KASAN_KMALLOC_REDZONE);
+}
#endif

void kasan_cache_shrink(struct kmem_cache *cache)
@@ -431,13 +460,6 @@ void kasan_poison_object_data(struct kmem_cache *cache, void *object)
kasan_poison_shadow(object,
round_up(cache->object_size, KASAN_SHADOW_SCALE_SIZE),
KASAN_KMALLOC_REDZONE);
-#ifdef CONFIG_SLAB
- if (cache->flags & SLAB_KASAN) {
- struct kasan_alloc_meta *alloc_info =
- get_alloc_info(cache, object);
- alloc_info->state = KASAN_STATE_INIT;
- }
-#endif
}

#ifdef CONFIG_SLAB
@@ -520,35 +542,41 @@ void kasan_poison_slab_free(struct kmem_cache *cache, void *object)
kasan_poison_shadow(object, rounded_up_size, KASAN_KMALLOC_FREE);
}

-bool kasan_slab_free(struct kmem_cache *cache, void *object)
+bool kasan_slab_free(struct kmem_cache *cache, void *object,
+ unsigned long caller)
{
#ifdef CONFIG_SLAB
+ struct kasan_alloc_meta *alloc_info;
+ struct kasan_free_meta *free_info;
+
/* RCU slabs could be legally used after free within the RCU period */
if (unlikely(cache->flags & SLAB_DESTROY_BY_RCU))
return false;

- if (likely(cache->flags & SLAB_KASAN)) {
- struct kasan_alloc_meta *alloc_info =
- get_alloc_info(cache, object);
- struct kasan_free_meta *free_info =
- get_free_info(cache, object);
-
- switch (alloc_info->state) {
- case KASAN_STATE_ALLOC:
- alloc_info->state = KASAN_STATE_QUARANTINE;
- quarantine_put(free_info, cache);
- set_track(&free_info->track, GFP_NOWAIT);
- kasan_poison_slab_free(cache, object);
- return true;
+ if (unlikely(!(cache->flags & SLAB_KASAN)))
+ return false;
+
+ alloc_info = get_alloc_info(cache, object);
+ kasan_meta_lock(alloc_info);
+ if (alloc_info->state == KASAN_STATE_ALLOC) {
+ free_info = get_free_info(cache, object);
+ quarantine_put(free_info, cache);
+ set_track(&free_info->track, GFP_NOWAIT);
+ kasan_poison_slab_free(cache, object);
+ alloc_info->state = KASAN_STATE_QUARANTINE;
+ kasan_meta_unlock(alloc_info);
+ return true;
+ }
+ switch (alloc_info->state) {
case KASAN_STATE_QUARANTINE:
case KASAN_STATE_FREE:
- pr_err("Double free");
- dump_stack();
- break;
+ kasan_report((unsigned long)object, 0, false, caller);
+ kasan_meta_unlock(alloc_info);
+ return true;
default:
break;
- }
}
+ kasan_meta_unlock(alloc_info);
return false;
#else
kasan_poison_slab_free(cache, object);
@@ -580,10 +608,16 @@ void kasan_kmalloc(struct kmem_cache *cache, const void *object, size_t size,
if (cache->flags & SLAB_KASAN) {
struct kasan_alloc_meta *alloc_info =
get_alloc_info(cache, object);
+ unsigned long flags;

+ local_irq_save(flags);
+ kasan_meta_lock(alloc_info);
alloc_info->state = KASAN_STATE_ALLOC;
- alloc_info->alloc_size = size;
+ alloc_info->size_delta = cache->object_size - size;
set_track(&alloc_info->track, flags);
+ kasan_unmark_bad_meta(alloc_info);
+ kasan_meta_unlock(alloc_info);
+ local_irq_restore(flags);
}
#endif
}
@@ -636,7 +670,7 @@ void kasan_kfree(void *ptr)
kasan_poison_shadow(ptr, PAGE_SIZE << compound_order(page),
KASAN_FREE_PAGE);
else
- kasan_slab_free(page->slab_cache, ptr);
+ kasan_slab_free(page->slab_cache, ptr, _RET_IP_);
}

void kasan_kfree_large(const void *ptr)
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index fb87923..ceaf016 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -3,12 +3,14 @@

#include <linux/kasan.h>
#include <linux/stackdepot.h>
+#include <linux/bit_spinlock.h>

#define KASAN_SHADOW_SCALE_SIZE (1UL << KASAN_SHADOW_SCALE_SHIFT)
#define KASAN_SHADOW_MASK (KASAN_SHADOW_SCALE_SIZE - 1)

#define KASAN_FREE_PAGE 0xFF /* page was freed */
#define KASAN_PAGE_REDZONE 0xFE /* redzone for kmalloc_large allocations */
+#define KASAN_KMALLOC_BAD_META 0xFD /* slab object header was overwritten */
#define KASAN_KMALLOC_REDZONE 0xFC /* redzone inside slub object */
#define KASAN_KMALLOC_FREE 0xFB /* object was freed (kmem_cache_free/kfree) */
#define KASAN_GLOBAL_REDZONE 0xFA /* redzone for global variable */
@@ -74,9 +76,17 @@ struct kasan_track {
};

struct kasan_alloc_meta {
+ union {
+ u64 data;
+ struct {
+ u32 lock : 1; /* lock bit */
+ u32 state : 2; /* enum kasan_state */
+ u32 size_delta : 23; /* object_size - alloc size */
+ u32 unused1 : 6;
+ u32 unused2;
+ };
+ };
struct kasan_track track;
- u32 state : 2; /* enum kasan_state */
- u32 alloc_size : 30;
};

struct qlist_node {
@@ -114,6 +124,36 @@ void kasan_report(unsigned long addr, size_t size,
void quarantine_put(struct kasan_free_meta *info, struct kmem_cache *cache);
void quarantine_reduce(void);
void quarantine_remove_cache(struct kmem_cache *cache);
+
+/* acquire per-object lock for access to KASAN metadata. */
+static inline void kasan_meta_lock(struct kasan_alloc_meta *alloc_info)
+{
+ unsigned long *lockp = (unsigned long *)&alloc_info->data;
+
+ while (unlikely(!bit_spin_trylock(0, lockp))) {
+ u8 *shadow = (u8 *)kasan_mem_to_shadow((void *)lockp);
+
+ if (READ_ONCE(*shadow) == KASAN_KMALLOC_BAD_META) {
+ /*
+ * a prior out-of-bounds access overwrote object header,
+ * flipping lock bit; break out to allow deallocation.
+ */
+ preempt_disable();
+ return;
+ }
+ while (test_bit(0, lockp))
+ cpu_relax();
+ }
+}
+
+/* release lock after a kasan_meta_lock(). */
+static inline void kasan_meta_unlock(struct kasan_alloc_meta *alloc_info)
+{
+ __bit_spin_unlock(0, (unsigned long *)&alloc_info->data);
+}
+
+void kasan_mark_bad_meta(struct kasan_alloc_meta *alloc_info,
+ struct kasan_access_info *info);
#else
static inline void quarantine_put(struct kasan_free_meta *info,
struct kmem_cache *cache) { }
diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
index 4973505..c4d45cb 100644
--- a/mm/kasan/quarantine.c
+++ b/mm/kasan/quarantine.c
@@ -148,7 +148,9 @@ static void qlink_free(struct qlist_node *qlink, struct kmem_cache *cache)
unsigned long flags;

local_irq_save(flags);
+ kasan_meta_lock(alloc_info);
alloc_info->state = KASAN_STATE_FREE;
+ kasan_meta_unlock(alloc_info);
___cache_free(cache, object, _THIS_IP_);
local_irq_restore(flags);
}
diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index b3c122d..4d0d70d 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -53,6 +53,17 @@ static void print_error_description(struct kasan_access_info *info)
const char *bug_type = "unknown-crash";
u8 *shadow_addr;

+#ifdef CONFIG_SLAB
+ if (!info->access_size) {
+ bug_type = "double-free";
+ pr_err("BUG: KASAN: %s attempt in %pS on object at addr %p\n",
+ bug_type, (void *)info->ip, info->access_addr);
+ pr_err("%s by task %s/%d\n", bug_type, current->comm,
+ task_pid_nr(current));
+ info->first_bad_addr = info->access_addr;
+ return;
+ }
+#endif
info->first_bad_addr = find_first_bad_addr(info->access_addr,
info->access_size);

@@ -75,6 +86,7 @@ static void print_error_description(struct kasan_access_info *info)
break;
case KASAN_PAGE_REDZONE:
case KASAN_KMALLOC_REDZONE:
+ case KASAN_KMALLOC_BAD_META:
bug_type = "slab-out-of-bounds";
break;
case KASAN_GLOBAL_REDZONE:
@@ -131,7 +143,7 @@ static void print_track(struct kasan_track *track)
}

static void object_err(struct kmem_cache *cache, struct page *page,
- void *object, char *unused_reason)
+ void *object, struct kasan_access_info *info)
{
struct kasan_alloc_meta *alloc_info = get_alloc_info(cache, object);
struct kasan_free_meta *free_info;
@@ -140,20 +152,22 @@ static void object_err(struct kmem_cache *cache, struct page *page,
pr_err("Object at %p, in cache %s\n", object, cache->name);
if (!(cache->flags & SLAB_KASAN))
return;
+ if (info->access_size)
+ kasan_meta_lock(alloc_info);
switch (alloc_info->state) {
case KASAN_STATE_INIT:
pr_err("Object not allocated yet\n");
break;
case KASAN_STATE_ALLOC:
pr_err("Object allocated with size %u bytes.\n",
- alloc_info->alloc_size);
+ (cache->object_size - alloc_info->size_delta));
pr_err("Allocation:\n");
print_track(&alloc_info->track);
break;
case KASAN_STATE_FREE:
case KASAN_STATE_QUARANTINE:
pr_err("Object freed, allocated with size %u bytes\n",
- alloc_info->alloc_size);
+ (cache->object_size - alloc_info->size_delta));
free_info = get_free_info(cache, object);
pr_err("Allocation:\n");
print_track(&alloc_info->track);
@@ -161,6 +175,10 @@ static void object_err(struct kmem_cache *cache, struct page *page,
print_track(&free_info->track);
break;
}
+ if (info->access_size) {
+ kasan_mark_bad_meta(alloc_info, info);
+ kasan_meta_unlock(alloc_info);
+ }
}
#endif

@@ -177,8 +195,12 @@ 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);
+#ifdef CONFIG_SLAB
+ object_err(cache, page, object, info);
+#else
object_err(cache, page, object,
"kasan: bad access detected");
+#endif
return;
}
dump_page(page, "kasan: bad access detected");
diff --git a/mm/slab.c b/mm/slab.c
index cc8bbc1..f7addb3 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2651,6 +2651,7 @@ static void cache_init_objs(struct kmem_cache *cachep,
cachep->ctor(objp);
kasan_poison_object_data(cachep, objp);
}
+ kasan_init_object(cachep, index_to_obj(cachep, page, i));

if (!shuffled)
set_free_obj(page, i, i);
@@ -3548,7 +3549,7 @@ static inline void __cache_free(struct kmem_cache *cachep, void *objp,
unsigned long caller)
{
/* Put the object into the quarantine, don't touch it for now. */
- if (kasan_slab_free(cachep, objp))
+ if (kasan_slab_free(cachep, objp, _RET_IP_))
return;

___cache_free(cachep, objp, caller);
diff --git a/mm/slub.c b/mm/slub.c
index 825ff45..21c2b78 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1344,7 +1344,7 @@ static inline void slab_free_hook(struct kmem_cache *s, void *x)
if (!(s->flags & SLAB_DEBUG_OBJECTS))
debug_check_no_obj_freed(x, s->object_size);

- kasan_slab_free(s, x);
+ kasan_slab_free(s, x, _RET_IP_);
}

static inline void slab_free_freelist_hook(struct kmem_cache *s,
--
1.7.1