Re: [PATCH v2 1/2] mm, kasan: improve double-free detection

From: Dmitry Vyukov
Date: Mon May 09 2016 - 06:31:43 EST

On Mon, May 9, 2016 at 12:26 PM, Andrey Ryabinin
<aryabinin@xxxxxxxxxxxxx> wrote:
>
>
> On 05/06/2016 02:47 PM, Kuthonuzo Luruo wrote:
>> 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().
>>
>> 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 'test_kasan' kasan_double_free() test in accompanying
>> patch.
>>
>> Signed-off-by: Kuthonuzo Luruo <kuthonuzo.luruo@xxxxxxx>
>> ---
>>
>> Changes in v2:
>> - Incorporated suggestions from Dmitry Vyukov. New per-object metadata
>> lock/unlock functions; kasan_alloc_meta modified to add new state while
>> using fewer bits overall.
>> - Double-free pr_err promoted to kasan_report().
>> - kasan_init_object() introduced to initialize KASAN object metadata
>> during slab creation. KASAN_STATE_INIT initialization removed from
>> kasan_poison_object_data().
>>
>> ---
>> include/linux/kasan.h | 8 +++
>> mm/kasan/kasan.c | 118 ++++++++++++++++++++++++++++++++++++-------------
>> mm/kasan/kasan.h | 15 +++++-
>> mm/kasan/quarantine.c | 7 +++-
>> mm/kasan/report.c | 31 +++++++++++--
>> mm/slab.c | 1 +
>> 6 files changed, 142 insertions(+), 38 deletions(-)
>>
>
> Sorry, but this patch is crap.
>
> Something like this, will fix the race:
>
> ---
> mm/kasan/kasan.c | 20 ++++----------------
> mm/kasan/kasan.h | 10 +++-------
> mm/kasan/quarantine.c | 1 -
> mm/kasan/report.c | 11 ++---------
> 4 files changed, 9 insertions(+), 33 deletions(-)
>
> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> index ef2e87b..8d078dc 100644
> --- a/mm/kasan/kasan.c
> +++ b/mm/kasan/kasan.c
> @@ -419,13 +419,6 @@ 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
> @@ -521,20 +514,15 @@ bool kasan_slab_free(struct kmem_cache *cache, void *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;
> + if (test_and_clear_bit(KASAN_STATE_ALLOCATED,
> + &alloc_info->state)) {
> quarantine_put(free_info, cache);
> set_track(&free_info->track, GFP_NOWAIT);
> kasan_poison_slab_free(cache, object);
> return true;
> - case KASAN_STATE_QUARANTINE:
> - case KASAN_STATE_FREE:
> + } else {
> pr_err("Double free");
> dump_stack();
> - break;
> - default:
> - break;
> }
> }
> return false;
> @@ -571,7 +559,7 @@ void kasan_kmalloc(struct kmem_cache *cache, const void *object, size_t size,
> struct kasan_alloc_meta *alloc_info =
> get_alloc_info(cache, object);
>
> - alloc_info->state = KASAN_STATE_ALLOC;
> + set_bit(KASAN_STATE_ALLOCATED, &alloc_info->state);
> alloc_info->alloc_size = size;
> set_track(&alloc_info->track, flags);
> }
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 7da78a6..2dcdc8f 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -60,10 +60,7 @@ struct kasan_global {
> */
>
> enum kasan_state {
> - KASAN_STATE_INIT,
> - KASAN_STATE_ALLOC,
> - KASAN_STATE_QUARANTINE,
> - KASAN_STATE_FREE
> + KASAN_STATE_ALLOCATED,
> };
>
> #define KASAN_STACK_DEPTH 64
> @@ -75,9 +72,8 @@ struct kasan_track {
>
> struct kasan_alloc_meta {
> struct kasan_track track;
> - u32 state : 2; /* enum kasan_state */
> - u32 alloc_size : 30;
> - u32 reserved;
> + unsigned long state;
> + u32 alloc_size;
> };
>
> struct kasan_free_meta {
> diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
> index 40159a6..ca33fd3 100644
> --- a/mm/kasan/quarantine.c
> +++ b/mm/kasan/quarantine.c
> @@ -147,7 +147,6 @@ static void qlink_free(void **qlink, struct kmem_cache *cache)
> unsigned long flags;
>
> local_irq_save(flags);
> - alloc_info->state = KASAN_STATE_FREE;
> ___cache_free(cache, object, _THIS_IP_);
> local_irq_restore(flags);
> }
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index b3c122d..c2b0e51 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -140,18 +140,12 @@ 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;
> - switch (alloc_info->state) {
> - case KASAN_STATE_INIT:
> - pr_err("Object not allocated yet\n");
> - break;
> - case KASAN_STATE_ALLOC:
> + if (test_bit(KASAN_STATE_ALLOCATED, &alloc_info->state)) {
> pr_err("Object allocated with size %u bytes.\n",
> alloc_info->alloc_size);
> pr_err("Allocation:\n");
> print_track(&alloc_info->track);

alloc_info->track is not necessary initialized when
KASAN_STATE_ALLOCATED is set. Worse, it can be initialized to a wrong
stack.


> - break;
> - case KASAN_STATE_FREE:
> - case KASAN_STATE_QUARANTINE:
> + } else {
> pr_err("Object freed, allocated with size %u bytes\n",
> alloc_info->alloc_size);
> free_info = get_free_info(cache, object);
> @@ -159,7 +153,6 @@ static void object_err(struct kmem_cache *cache, struct page *page,
> print_track(&alloc_info->track);
> pr_err("Deallocation:\n");
> print_track(&free_info->track);
> - break;
> }
> }
> #endif
> --
> 2.7.3
>