Re: [PATCH v3 2/2] slub: Introduce CONFIG_SLUB_RCU_DEBUG

[Vlastimil Babka]

On 7/25/24 5:31 PM, Jann Horn wrote:
> Currently, KASAN is unable to catch use-after-free in SLAB_TYPESAFE_BY_RCU
> slabs because use-after-free is allowed within the RCU grace period by
> design.
> 
> Add a SLUB debugging feature which RCU-delays every individual
> kmem_cache_free() before either actually freeing the object or handing it
> off to KASAN, and change KASAN to poison freed objects as normal when this
> option is enabled.
> 
> For now I've configured Kconfig.debug to default-enable this feature in the
> KASAN GENERIC and SW_TAGS modes; I'm not enabling it by default in HW_TAGS
> mode because I'm not sure if it might have unwanted performance degradation
> effects there.
> 
> Note that this is mostly useful with KASAN in the quarantine-based GENERIC
> mode; SLAB_TYPESAFE_BY_RCU slabs are basically always also slabs with a
> ->ctor, and KASAN's assign_tag() currently has to assign fixed tags for
> those, reducing the effectiveness of SW_TAGS/HW_TAGS mode.
> (A possible future extension of this work would be to also let SLUB call
> the ->ctor() on every allocation instead of only when the slab page is
> allocated; then tag-based modes would be able to assign new tags on every
> reallocation.)
> 
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>

Yeah this "we try but might fail" looks to be a suitable tradeoff for this
debuggin feature in that it keeps the complexity lower. Thanks.

Acked-by: Vlastimil Babka <vbabka@xxxxxxx> #slab

> ---
>  include/linux/kasan.h | 14 ++++++----
>  mm/Kconfig.debug      | 29 ++++++++++++++++++++
>  mm/kasan/common.c     | 13 +++++----
>  mm/kasan/kasan_test.c | 44 +++++++++++++++++++++++++++++
>  mm/slab_common.c      | 12 ++++++++
>  mm/slub.c             | 76 +++++++++++++++++++++++++++++++++++++++++++++------
>  6 files changed, 170 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index ebd93c843e78..c64483d3e2bd 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -186,12 +186,15 @@ static __always_inline bool kasan_slab_pre_free(struct kmem_cache *s,
>  }
>  
>  bool __kasan_slab_free(struct kmem_cache *s, void *object,
> -			unsigned long ip, bool init);
> +			unsigned long ip, bool init, bool after_rcu_delay);
>  static __always_inline bool kasan_slab_free(struct kmem_cache *s,
> -						void *object, bool init)
> +						void *object, bool init,
> +						bool after_rcu_delay)
>  {
> -	if (kasan_enabled())
> -		return __kasan_slab_free(s, object, _RET_IP_, init);
> +	if (kasan_enabled()) {
> +		return __kasan_slab_free(s, object, _RET_IP_, init,
> +				after_rcu_delay);
> +	}
>  	return false;
>  }
>  
> @@ -387,7 +390,8 @@ static inline bool kasan_slab_pre_free(struct kmem_cache *s, void *object)
>  	return false;
>  }
>  
> -static inline bool kasan_slab_free(struct kmem_cache *s, void *object, bool init)
> +static inline bool kasan_slab_free(struct kmem_cache *s, void *object,
> +				   bool init, bool after_rcu_delay)
>  {
>  	return false;
>  }
> diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
> index afc72fde0f03..0c088532f5a7 100644
> --- a/mm/Kconfig.debug
> +++ b/mm/Kconfig.debug
> @@ -70,6 +70,35 @@ config SLUB_DEBUG_ON
>  	  off in a kernel built with CONFIG_SLUB_DEBUG_ON by specifying
>  	  "slab_debug=-".
>  
> +config SLUB_RCU_DEBUG
> +	bool "Make use-after-free detection possible in TYPESAFE_BY_RCU caches"
> +	depends on SLUB_DEBUG
> +	default KASAN_GENERIC || KASAN_SW_TAGS
> +	help
> +	  Make SLAB_TYPESAFE_BY_RCU caches behave approximately as if the cache
> +	  was not marked as SLAB_TYPESAFE_BY_RCU and every caller used
> +	  kfree_rcu() instead.
> +
> +	  This is intended for use in combination with KASAN, to enable KASAN to
> +	  detect use-after-free accesses in such caches.
> +	  (KFENCE is able to do that independent of this flag.)
> +
> +	  This might degrade performance.
> +	  Unfortunately this also prevents a very specific bug pattern from
> +	  triggering (insufficient checks against an object being recycled
> +	  within the RCU grace period); so this option can be turned off even on
> +	  KASAN builds, in case you want to test for such a bug.
> +
> +	  If you're using this for testing bugs / fuzzing and care about
> +	  catching all the bugs WAY more than performance, you might want to
> +	  also turn on CONFIG_RCU_STRICT_GRACE_PERIOD.
> +
> +	  WARNING:
> +	  This is designed as a debugging feature, not a security feature.
> +	  Objects are sometimes recycled without RCU delay under memory pressure.
> +
> +	  If unsure, say N.
> +
>  config PAGE_OWNER
>  	bool "Track page owner"
>  	depends on DEBUG_KERNEL && STACKTRACE_SUPPORT
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 7c7fc6ce7eb7..d92cb2e9189d 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -238,7 +238,8 @@ static enum free_validation_result check_slab_free(struct kmem_cache *cache,
>  }
>  
>  static inline bool poison_slab_object(struct kmem_cache *cache, void *object,
> -				      unsigned long ip, bool init)
> +				      unsigned long ip, bool init,
> +				      bool after_rcu_delay)
>  {
>  	void *tagged_object = object;
>  	enum free_validation_result valid = check_slab_free(cache, object, ip);
> @@ -251,7 +252,8 @@ static inline bool poison_slab_object(struct kmem_cache *cache, void *object,
>  	object = kasan_reset_tag(object);
>  
>  	/* RCU slabs could be legally used after free within the RCU period. */
> -	if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU))
> +	if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU) &&
> +	    !after_rcu_delay)
>  		return false;
>  
>  	kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
> @@ -270,7 +272,8 @@ bool __kasan_slab_pre_free(struct kmem_cache *cache, void *object,
>  }
>  
>  bool __kasan_slab_free(struct kmem_cache *cache, void *object,
> -				unsigned long ip, bool init)
> +				unsigned long ip, bool init,
> +				bool after_rcu_delay)
>  {
>  	if (is_kfence_address(object))
>  		return false;
> @@ -280,7 +283,7 @@ bool __kasan_slab_free(struct kmem_cache *cache, void *object,
>  	 * freelist. The object will thus never be allocated again and its
>  	 * metadata will never get released.
>  	 */
> -	if (poison_slab_object(cache, object, ip, init))
> +	if (poison_slab_object(cache, object, ip, init, after_rcu_delay))
>  		return true;
>  
>  	/*
> @@ -535,7 +538,7 @@ bool __kasan_mempool_poison_object(void *ptr, unsigned long ip)
>  		return false;
>  
>  	slab = folio_slab(folio);
> -	return !poison_slab_object(slab->slab_cache, ptr, ip, false);
> +	return !poison_slab_object(slab->slab_cache, ptr, ip, false, false);
>  }
>  
>  void __kasan_mempool_unpoison_object(void *ptr, size_t size, unsigned long ip)
> diff --git a/mm/kasan/kasan_test.c b/mm/kasan/kasan_test.c
> index 7b32be2a3cf0..cba782a4b072 100644
> --- a/mm/kasan/kasan_test.c
> +++ b/mm/kasan/kasan_test.c
> @@ -996,6 +996,49 @@ static void kmem_cache_invalid_free(struct kunit *test)
>  	kmem_cache_destroy(cache);
>  }
>  
> +static void kmem_cache_rcu_uaf(struct kunit *test)
> +{
> +	char *p;
> +	size_t size = 200;
> +	struct kmem_cache *cache;
> +
> +	KASAN_TEST_NEEDS_CONFIG_ON(test, CONFIG_SLUB_RCU_DEBUG);
> +
> +	cache = kmem_cache_create("test_cache", size, 0, SLAB_TYPESAFE_BY_RCU,
> +				  NULL);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, cache);
> +
> +	p = kmem_cache_alloc(cache, GFP_KERNEL);
> +	if (!p) {
> +		kunit_err(test, "Allocation failed: %s\n", __func__);
> +		kmem_cache_destroy(cache);
> +		return;
> +	}
> +	*p = 1;
> +
> +	rcu_read_lock();
> +
> +	/* Free the object - this will internally schedule an RCU callback. */
> +	kmem_cache_free(cache, p);
> +
> +	/* We should still be allowed to access the object at this point because
> +	 * the cache is SLAB_TYPESAFE_BY_RCU and we've been in an RCU read-side
> +	 * critical section since before the kmem_cache_free().
> +	 */
> +	READ_ONCE(*p);
> +
> +	rcu_read_unlock();
> +
> +	/* Wait for the RCU callback to execute; after this, the object should
> +	 * have actually been freed from KASAN's perspective.
> +	 */
> +	rcu_barrier();
> +
> +	KUNIT_EXPECT_KASAN_FAIL(test, READ_ONCE(*p));
> +
> +	kmem_cache_destroy(cache);
> +}
> +
>  static void empty_cache_ctor(void *object) { }
>  
>  static void kmem_cache_double_destroy(struct kunit *test)
> @@ -1937,6 +1980,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
>  	KUNIT_CASE(kmem_cache_oob),
>  	KUNIT_CASE(kmem_cache_double_free),
>  	KUNIT_CASE(kmem_cache_invalid_free),
> +	KUNIT_CASE(kmem_cache_rcu_uaf),
>  	KUNIT_CASE(kmem_cache_double_destroy),
>  	KUNIT_CASE(kmem_cache_accounted),
>  	KUNIT_CASE(kmem_cache_bulk),
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 1560a1546bb1..19511e34017b 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -450,6 +450,18 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
>  
>  static int shutdown_cache(struct kmem_cache *s)
>  {
> +	if (IS_ENABLED(CONFIG_SLUB_RCU_DEBUG) &&
> +	    (s->flags & SLAB_TYPESAFE_BY_RCU)) {
> +		/*
> +		 * Under CONFIG_SLUB_RCU_DEBUG, when objects in a
> +		 * SLAB_TYPESAFE_BY_RCU slab are freed, SLUB will internally
> +		 * defer their freeing with call_rcu().
> +		 * Wait for such call_rcu() invocations here before actually
> +		 * destroying the cache.
> +		 */
> +		rcu_barrier();
> +	}
> +
>  	/* free asan quarantined objects */
>  	kasan_cache_shutdown(s);
>  
> diff --git a/mm/slub.c b/mm/slub.c
> index 34724704c52d..f44eec209e3e 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2144,15 +2144,26 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
>  }
>  #endif /* CONFIG_MEMCG_KMEM */
>  
> +#ifdef CONFIG_SLUB_RCU_DEBUG
> +static void slab_free_after_rcu_debug(struct rcu_head *rcu_head);
> +
> +struct rcu_delayed_free {
> +	struct rcu_head head;
> +	void *object;
> +};
> +#endif
> +
>  /*
>   * Hooks for other subsystems that check memory allocations. In a typical
>   * production configuration these hooks all should produce no code at all.
>   *
>   * Returns true if freeing of the object can proceed, false if its reuse
> - * was delayed by KASAN quarantine, or it was returned to KFENCE.
> + * was delayed by CONFIG_SLUB_RCU_DEBUG or KASAN quarantine, or it was returned
> + * to KFENCE.
>   */
>  static __always_inline
> -bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
> +bool slab_free_hook(struct kmem_cache *s, void *x, bool init,
> +		    bool after_rcu_delay)
>  {
>  	kmemleak_free_recursive(x, s->flags);
>  	kmsan_slab_free(s, x);
> @@ -2163,7 +2174,7 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
>  		debug_check_no_obj_freed(x, s->object_size);
>  
>  	/* Use KCSAN to help debug racy use-after-free. */
> -	if (!(s->flags & SLAB_TYPESAFE_BY_RCU))
> +	if (!(s->flags & SLAB_TYPESAFE_BY_RCU) || after_rcu_delay)
>  		__kcsan_check_access(x, s->object_size,
>  				     KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT);
>  
> @@ -2177,6 +2188,28 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
>  	if (kasan_slab_pre_free(s, x))
>  		return false;
>  
> +#ifdef CONFIG_SLUB_RCU_DEBUG
> +	if ((s->flags & SLAB_TYPESAFE_BY_RCU) && !after_rcu_delay) {
> +		struct rcu_delayed_free *delayed_free;
> +
> +		delayed_free = kmalloc(sizeof(*delayed_free), GFP_NOWAIT);
> +		if (delayed_free) {
> +			/*
> +			 * Let KASAN track our call stack as a "related work
> +			 * creation", just like if the object had been freed
> +			 * normally via kfree_rcu().
> +			 * We have to do this manually because the rcu_head is
> +			 * not located inside the object.
> +			 */
> +			kasan_record_aux_stack_noalloc(x);
> +
> +			delayed_free->object = x;
> +			call_rcu(&delayed_free->head, slab_free_after_rcu_debug);
> +			return false;
> +		}
> +	}
> +#endif /* CONFIG_SLUB_RCU_DEBUG */
> +
>  	/*
>  	 * As memory initialization might be integrated into KASAN,
>  	 * kasan_slab_free and initialization memset's must be
> @@ -2200,7 +2233,7 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
>  		       s->size - inuse - rsize);
>  	}
>  	/* KASAN might put x into memory quarantine, delaying its reuse. */
> -	return !kasan_slab_free(s, x, init);
> +	return !kasan_slab_free(s, x, init, after_rcu_delay);
>  }
>  
>  static __fastpath_inline
> @@ -2214,7 +2247,7 @@ bool slab_free_freelist_hook(struct kmem_cache *s, void **head, void **tail,
>  	bool init;
>  
>  	if (is_kfence_address(next)) {
> -		slab_free_hook(s, next, false);
> +		slab_free_hook(s, next, false, false);
>  		return false;
>  	}
>  
> @@ -2229,7 +2262,7 @@ bool slab_free_freelist_hook(struct kmem_cache *s, void **head, void **tail,
>  		next = get_freepointer(s, object);
>  
>  		/* If object's reuse doesn't have to be delayed */
> -		if (likely(slab_free_hook(s, object, init))) {
> +		if (likely(slab_free_hook(s, object, init, false))) {
>  			/* Move object to the new freelist */
>  			set_freepointer(s, object, *head);
>  			*head = object;
> @@ -4442,7 +4475,7 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
>  	memcg_slab_free_hook(s, slab, &object, 1);
>  	alloc_tagging_slab_free_hook(s, slab, &object, 1);
>  
> -	if (likely(slab_free_hook(s, object, slab_want_init_on_free(s))))
> +	if (likely(slab_free_hook(s, object, slab_want_init_on_free(s), false)))
>  		do_slab_free(s, slab, object, object, 1, addr);
>  }
>  
> @@ -4451,7 +4484,7 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
>  static noinline
>  void memcg_alloc_abort_single(struct kmem_cache *s, void *object)
>  {
> -	if (likely(slab_free_hook(s, object, slab_want_init_on_free(s))))
> +	if (likely(slab_free_hook(s, object, slab_want_init_on_free(s), false)))
>  		do_slab_free(s, virt_to_slab(object), object, object, 1, _RET_IP_);
>  }
>  #endif
> @@ -4470,6 +4503,33 @@ void slab_free_bulk(struct kmem_cache *s, struct slab *slab, void *head,
>  		do_slab_free(s, slab, head, tail, cnt, addr);
>  }
>  
> +#ifdef CONFIG_SLUB_RCU_DEBUG
> +static void slab_free_after_rcu_debug(struct rcu_head *rcu_head)
> +{
> +	struct rcu_delayed_free *delayed_free =
> +			container_of(rcu_head, struct rcu_delayed_free, head);
> +	void *object = delayed_free->object;
> +	struct slab *slab = virt_to_slab(object);
> +	struct kmem_cache *s;
> +
> +	if (WARN_ON(is_kfence_address(rcu_head)))
> +		return;
> +
> +	/* find the object and the cache again */
> +	if (WARN_ON(!slab))
> +		return;
> +	s = slab->slab_cache;
> +	if (WARN_ON(!(s->flags & SLAB_TYPESAFE_BY_RCU)))
> +		return;
> +
> +	/* resume freeing */
> +	if (!slab_free_hook(s, object, slab_want_init_on_free(s), true))
> +		return;
> +	do_slab_free(s, slab, object, NULL, 1, _THIS_IP_);
> +	kfree(delayed_free);
> +}
> +#endif /* CONFIG_SLUB_RCU_DEBUG */
> +
>  #ifdef CONFIG_KASAN_GENERIC
>  void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr)
>  {
>