On Fri, 2 Aug 2024 at 22:32, Jann Horn <jannh@xxxxxxxxxx> 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.)
>
> Tested-by: syzbot+263726e59eab6b442723@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Acked-by: Marco Elver <elver@xxxxxxxxxx>
Looks good - let's see what the fuzzers will find with it. :-)
Feel free to ignore the below comments if there isn't a v+1.
> ---
> include/linux/kasan.h | 17 +++++++----
> mm/Kconfig.debug | 30 +++++++++++++++++++
> mm/kasan/common.c | 11 +++----
> mm/kasan/kasan_test.c | 46 ++++++++++++++++++++++++++++++
> mm/slab_common.c | 12 ++++++++
> mm/slub.c | 79 +++++++++++++++++++++++++++++++++++++++++++++------
> 6 files changed, 176 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 1570c7191176..00a3bf7c0d8f 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -193,40 +193,44 @@ static __always_inline bool kasan_slab_pre_free(struct kmem_cache *s,
> {
> if (kasan_enabled())
> return __kasan_slab_pre_free(s, object, _RET_IP_);
> return false;
> }
>
> -bool __kasan_slab_free(struct kmem_cache *s, void *object, bool init);
> +bool __kasan_slab_free(struct kmem_cache *s, void *object, bool init,
> + bool still_accessible);
> /**
> * kasan_slab_free - Poison, initialize, and quarantine a slab object.
> * @object: Object to be freed.
> * @init: Whether to initialize the object.
> + * @still_accessible: Whether the object contents are still accessible.
> *
> * This function informs that a slab object has been freed and is not
> - * supposed to be accessed anymore, except for objects in
> - * SLAB_TYPESAFE_BY_RCU caches.
> + * supposed to be accessed anymore, except when @still_accessible is set
> + * (indicating that the object is in a SLAB_TYPESAFE_BY_RCU cache and an RCU
> + * grace period might not have passed yet).
> *
> * For KASAN modes that have integrated memory initialization
> * (kasan_has_integrated_init() == true), this function also initializes
> * the object's memory. For other modes, the @init argument is ignored.
> *
> * This function might also take ownership of the object to quarantine it.
> * When this happens, KASAN will defer freeing the object to a later
> * stage and handle it internally until then. The return value indicates
> * whether KASAN took ownership of the object.
> *
> * This function is intended only for use by the slab allocator.
> *
> * @Return true if KASAN took ownership of the object; false otherwise.
> */
> static __always_inline bool kasan_slab_free(struct kmem_cache *s,
> - void *object, bool init)
> + void *object, bool init,
> + bool still_accessible)
> {
> if (kasan_enabled())
> - return __kasan_slab_free(s, object, init);
> + return __kasan_slab_free(s, object, init, still_accessible);
> return false;
> }
>
> void __kasan_kfree_large(void *ptr, unsigned long ip);
> static __always_inline void kasan_kfree_large(void *ptr)
> {
> @@ -416,13 +420,14 @@ static inline void *kasan_init_slab_obj(struct kmem_cache *cache,
>
> 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 still_accessible)
> {
> return false;
> }
> static inline void kasan_kfree_large(void *ptr) {}
> static inline void *kasan_slab_alloc(struct kmem_cache *s, void *object,
> gfp_t flags, bool init)
> diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
> index afc72fde0f03..8e440214aac8 100644
> --- a/mm/Kconfig.debug
> +++ b/mm/Kconfig.debug
> @@ -67,12 +67,42 @@ config SLUB_DEBUG_ON
> equivalent to specifying the "slab_debug" parameter on boot.
> There is no support for more fine grained debug control like
> possible with slab_debug=xxx. SLUB debugging may be switched
> off in a kernel built with CONFIG_SLUB_DEBUG_ON by specifying
> "slab_debug=-".
>
> +config SLUB_RCU_DEBUG
> + bool "Enable UAF detection in TYPESAFE_BY_RCU caches (for KASAN)"
> + depends on SLUB_DEBUG
> + depends on KASAN # not a real dependency; currently useless without KASAN
This comment is odd. If it's useless without KASAN then it definitely
depends on KASAN. I suppose the code compiles without KASAN, but I
think that's secondary.
> + 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
> select DEBUG_FS
> select STACKTRACE
> select STACKDEPOT
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index f26bbc087b3b..ed4873e18c75 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -227,43 +227,44 @@ static bool check_slab_allocation(struct kmem_cache *cache, void *object,
> }
>
> return false;
> }
>
> static inline void poison_slab_object(struct kmem_cache *cache, void *object,
> - bool init)
> + bool init, bool still_accessible)
> {
> void *tagged_object = 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(still_accessible))
> return;
>
> kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
> KASAN_SLAB_FREE, init);
>
> if (kasan_stack_collection_enabled())
> kasan_save_free_info(cache, tagged_object);
> }
>
> bool __kasan_slab_pre_free(struct kmem_cache *cache, void *object,
> unsigned long ip)
> {
> if (!kasan_arch_is_ready() || is_kfence_address(object))
> return false;
> return check_slab_allocation(cache, object, ip);
> }
>
> -bool __kasan_slab_free(struct kmem_cache *cache, void *object, bool init)
> +bool __kasan_slab_free(struct kmem_cache *cache, void *object, bool init,
> + bool still_accessible)
> {
> if (!kasan_arch_is_ready() || is_kfence_address(object))
> return false;
>
> - poison_slab_object(cache, object, init);
> + poison_slab_object(cache, object, init, still_accessible);
>
> /*
> * If the object is put into quarantine, do not let slab put the object
> * onto the freelist for now. The object's metadata is kept until the
> * object gets evicted from quarantine.
> */
> @@ -515,13 +516,13 @@ bool __kasan_mempool_poison_object(void *ptr, unsigned long ip)
>
> slab = folio_slab(folio);
>
> if (check_slab_allocation(slab->slab_cache, ptr, ip))
> return false;
>
> - poison_slab_object(slab->slab_cache, ptr, false);
> + poison_slab_object(slab->slab_cache, ptr, false, false);
> return true;
> }
>
> void __kasan_mempool_unpoison_object(void *ptr, size_t size, unsigned long ip)
> {
> struct slab *slab;
> diff --git a/mm/kasan/kasan_test.c b/mm/kasan/kasan_test.c
> index 7b32be2a3cf0..567d33b493e2 100644
> --- a/mm/kasan/kasan_test.c
> +++ b/mm/kasan/kasan_test.c
> @@ -993,12 +993,57 @@ static void kmem_cache_invalid_free(struct kunit *test)
> */
> kmem_cache_free(cache, p);
>
> 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)
> {
> struct kmem_cache *cache;
>
> @@ -1934,12 +1979,13 @@ static struct kunit_case kasan_kunit_test_cases[] = {
> KUNIT_CASE(workqueue_uaf),
> KUNIT_CASE(kfree_via_page),
> KUNIT_CASE(kfree_via_phys),
> 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),
> KUNIT_CASE(mempool_kmalloc_oob_right),
> KUNIT_CASE(mempool_kmalloc_large_oob_right),
> KUNIT_CASE(mempool_slab_oob_right),
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 40b582a014b8..df09066d56fe 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -539,12 +539,24 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
> kmem_cache_release(s);
> }
> }
>
> 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);
>
> if (__kmem_cache_shutdown(s) != 0)
> return -EBUSY;
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 0c98b6a2124f..a89f2006d46e 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2197,45 +2197,81 @@ static inline bool memcg_slab_post_alloc_hook(struct kmem_cache *s,
> static inline void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
> void **p, int objects)
> {
> }
> #endif /* CONFIG_MEMCG */
>
> +#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)
> {
> + /* Are the object contents still accessible? */
> + bool still_accessible = (s->flags & SLAB_TYPESAFE_BY_RCU) && !after_rcu_delay;
> +
> kmemleak_free_recursive(x, s->flags);
> kmsan_slab_free(s, x);
>
> debug_check_no_locks_freed(x, s->object_size);
>
> if (!(s->flags & SLAB_DEBUG_OBJECTS))
> 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 (!still_accessible)
> __kcsan_check_access(x, s->object_size,
> KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT);
>
> if (kfence_free(x))
> return false;
>
> /*
> * Give KASAN a chance to notice an invalid free operation before we
> * modify the object.
> */
> if (kasan_slab_pre_free(s, x))
> return false;
>
> +#ifdef CONFIG_SLUB_RCU_DEBUG
> + if (still_accessible) {
> + 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
> * kept together to avoid discrepancies in behavior.
> *
> * The initialization memset's clear the object and the metadata,
> @@ -2253,42 +2289,42 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
> memset(kasan_reset_tag(x), 0, s->object_size);
> rsize = (s->flags & SLAB_RED_ZONE) ? s->red_left_pad : 0;
> memset((char *)kasan_reset_tag(x) + inuse, 0,
> 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, still_accessible);
> }
>
> static __fastpath_inline
> bool slab_free_freelist_hook(struct kmem_cache *s, void **head, void **tail,
> int *cnt)
> {
>
> void *object;
> void *next = *head;
> void *old_tail = *tail;
> bool init;
>
> if (is_kfence_address(next)) {
> - slab_free_hook(s, next, false);
> + slab_free_hook(s, next, false, false);
> return false;
> }
>
> /* Head and tail of the reconstructed freelist */
> *head = NULL;
> *tail = NULL;
>
> init = slab_want_init_on_free(s);
>
> do {
> object = next;
> 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;
> if (!*tail)
> *tail = object;
> } else {
> @@ -4474,40 +4510,67 @@ static __fastpath_inline
> void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
> unsigned long addr)
> {
> 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);
> }
>
> #ifdef CONFIG_MEMCG
> /* Do not inline the rare memcg charging failed path into the allocation path */
> 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
>
> static __fastpath_inline
> void slab_free_bulk(struct kmem_cache *s, struct slab *slab, void *head,
> void *tail, void **p, int cnt, unsigned long addr)
> {
> memcg_slab_free_hook(s, slab, p, cnt);
> alloc_tagging_slab_free_hook(s, slab, p, cnt);
> /*
> * With KASAN enabled slab_free_freelist_hook modifies the freelist
> * to remove objects, whose reuse must be delayed.
> */
> if (likely(slab_free_freelist_hook(s, &head, &tail, &cnt)))
> 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);
Minor: Some of these line breaks are unnecessary (kernel allows 100+
cols) - but up to you if you want to change it.
> + void *object = delayed_free->object;
> + struct slab *slab = virt_to_slab(object);
> + struct kmem_cache *s;
> +
> + if (WARN_ON(is_kfence_address(object)))
> + 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, object, 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)
> {
> do_slab_free(cache, virt_to_slab(x), x, x, 1, addr);
> }
> #endif
>
> --
> 2.46.0.rc2.264.g509ed76dc8-goog
>
