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

["Luruo, Kuthonuzo" <kuthonuzo.luruo@hpe.com>]

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> > 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@hpe.com>
> > ---
> >
> > 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);
> -		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

This patch with atomic bit op is similar in spirit to v1 except that it increases metadata size.

Kuthonuzo
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