Message ID | 20240725-kasan-tsbrcu-v3-1-51c92f8f1101@google.com (mailing list archive) |
---|---|
State | New |
Headers | show |
Series | allow KASAN to detect UAF in SLAB_TYPESAFE_BY_RCU slabs | expand |
On Thu, Jul 25, 2024 at 5:32 PM Jann Horn <jannh@google.com> wrote: > > Currently, when KASAN is combined with init-on-free behavior, the > initialization happens before KASAN's "invalid free" checks. > > More importantly, a subsequent commit will want to use the object metadata > region to store an rcu_head, and we should let KASAN check that the object > pointer is valid before that. (Otherwise that change will make the existing > testcase kmem_cache_invalid_free fail.) This is not the case since v3, right? Do we still need this patch? If it's still needed, see the comment below. Thank you! > So add a new KASAN hook that allows KASAN to pre-validate a > kmem_cache_free() operation before SLUB actually starts modifying the > object or its metadata. > > Acked-by: Vlastimil Babka <vbabka@suse.cz> #slub > Signed-off-by: Jann Horn <jannh@google.com> > --- > include/linux/kasan.h | 16 ++++++++++++++++ > mm/kasan/common.c | 51 +++++++++++++++++++++++++++++++++++++++------------ > mm/slub.c | 7 +++++++ > 3 files changed, 62 insertions(+), 12 deletions(-) > > diff --git a/include/linux/kasan.h b/include/linux/kasan.h > index 70d6a8f6e25d..ebd93c843e78 100644 > --- a/include/linux/kasan.h > +++ b/include/linux/kasan.h > @@ -175,6 +175,16 @@ static __always_inline void * __must_check kasan_init_slab_obj( > return (void *)object; > } > > +bool __kasan_slab_pre_free(struct kmem_cache *s, void *object, > + unsigned long ip); > +static __always_inline bool kasan_slab_pre_free(struct kmem_cache *s, > + void *object) > +{ > + if (kasan_enabled()) > + return __kasan_slab_pre_free(s, object, _RET_IP_); > + return false; > +} Please add a documentation comment for this new hook; something like what we have for kasan_mempool_poison_pages() and some of the others. (I've been meaning to add them for all of them, but still didn't get around to that.) > + > bool __kasan_slab_free(struct kmem_cache *s, void *object, > unsigned long ip, bool init); > static __always_inline bool kasan_slab_free(struct kmem_cache *s, > @@ -371,6 +381,12 @@ static inline void *kasan_init_slab_obj(struct kmem_cache *cache, > { > return (void *)object; > } > + > +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) > { > return false; > diff --git a/mm/kasan/common.c b/mm/kasan/common.c > index 85e7c6b4575c..7c7fc6ce7eb7 100644 > --- a/mm/kasan/common.c > +++ b/mm/kasan/common.c > @@ -208,31 +208,52 @@ void * __must_check __kasan_init_slab_obj(struct kmem_cache *cache, > return (void *)object; > } > > -static inline bool poison_slab_object(struct kmem_cache *cache, void *object, > - unsigned long ip, bool init) > +enum free_validation_result { > + KASAN_FREE_IS_IGNORED, > + KASAN_FREE_IS_VALID, > + KASAN_FREE_IS_INVALID > +}; > + > +static enum free_validation_result check_slab_free(struct kmem_cache *cache, > + void *object, unsigned long ip) > { > - void *tagged_object; > + void *tagged_object = object; > > - if (!kasan_arch_is_ready()) > - return false; > + if (is_kfence_address(object) || !kasan_arch_is_ready()) > + return KASAN_FREE_IS_IGNORED; > > - tagged_object = object; > object = kasan_reset_tag(object); > > if (unlikely(nearest_obj(cache, virt_to_slab(object), object) != object)) { > kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_INVALID_FREE); > - return true; > + return KASAN_FREE_IS_INVALID; > } > > - /* RCU slabs could be legally used after free within the RCU period. */ > - if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU)) > - return false; > - > if (!kasan_byte_accessible(tagged_object)) { > kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_DOUBLE_FREE); > - return true; > + return KASAN_FREE_IS_INVALID; > } > > + return KASAN_FREE_IS_VALID; > +} > + > +static inline bool poison_slab_object(struct kmem_cache *cache, void *object, > + unsigned long ip, bool init) > +{ > + void *tagged_object = object; > + enum free_validation_result valid = check_slab_free(cache, object, ip); I believe we don't need check_slab_free() here, as it was already done in kasan_slab_pre_free()? Checking just kasan_arch_is_ready() and is_kfence_address() should save a bit on performance impact. Though if we remove check_slab_free() from here, we do need to add it to __kasan_mempool_poison_object(). > + > + if (valid == KASAN_FREE_IS_IGNORED) > + return false; > + if (valid == KASAN_FREE_IS_INVALID) > + return true; > + > + 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)) > + return false; I vaguely recall there was some reason why this check was done before the kasan_byte_accessible() check, but I might be wrong. Could you try booting the kernel with only this patch applied to see if anything breaks? > + > kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE), > KASAN_SLAB_FREE, init); > > @@ -242,6 +263,12 @@ static inline bool poison_slab_object(struct kmem_cache *cache, void *object, > return false; > } > > +bool __kasan_slab_pre_free(struct kmem_cache *cache, void *object, > + unsigned long ip) > +{ > + return check_slab_free(cache, object, ip) == KASAN_FREE_IS_INVALID; > +} > + > bool __kasan_slab_free(struct kmem_cache *cache, void *object, > unsigned long ip, bool init) > { > diff --git a/mm/slub.c b/mm/slub.c > index 4927edec6a8c..34724704c52d 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2170,6 +2170,13 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init) > 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; > + > /* > * As memory initialization might be integrated into KASAN, > * kasan_slab_free and initialization memset's must be > > -- > 2.45.2.1089.g2a221341d9-goog >
On Fri, Jul 26, 2024 at 2:43 AM Andrey Konovalov <andreyknvl@gmail.com> wrote: > On Thu, Jul 25, 2024 at 5:32 PM Jann Horn <jannh@google.com> wrote: > > > > Currently, when KASAN is combined with init-on-free behavior, the > > initialization happens before KASAN's "invalid free" checks. > > > > More importantly, a subsequent commit will want to use the object metadata > > region to store an rcu_head, and we should let KASAN check that the object > > pointer is valid before that. (Otherwise that change will make the existing > > testcase kmem_cache_invalid_free fail.) > > This is not the case since v3, right? Oh, you're right, this text is now wrong. > Do we still need this patch? I just tried removing this patch from the series; without it, the kmem_cache_invalid_free kunit test fails because the kmem_cache_free() no longer synchronously notices that the pointer is misaligned. I guess I could change the testcase like this to make the tests pass without this patch, but I'd like to hear from you or another KASAN person whether you think that's a reasonable change: diff --git a/mm/kasan/kasan_test.c b/mm/kasan/kasan_test.c index cba782a4b072..f44b0dcb0e84 100644 --- a/mm/kasan/kasan_test.c +++ b/mm/kasan/kasan_test.c @@ -981,14 +981,21 @@ static void kmem_cache_invalid_free(struct kunit *test) if (!p) { kunit_err(test, "Allocation failed: %s\n", __func__); kmem_cache_destroy(cache); return; } - /* Trigger invalid free, the object doesn't get freed. */ - KUNIT_EXPECT_KASAN_FAIL(test, kmem_cache_free(cache, p + 1)); + /* + * Trigger invalid free, the object doesn't get freed. + * Note that the invalid free detection may happen asynchronously + * under CONFIG_SLUB_RCU_DEBUG. + */ + KUNIT_EXPECT_KASAN_FAIL(test, ({ + kmem_cache_free(cache, p + 1); + rcu_barrier(); + })); Being able to get rid of this patch would be a nice simplification, so if you think asynchronous invalid-free detection for TYPESAFE_BY_RCU slabs is fine, I'll happily throw it out.
On Fri, Jul 26, 2024 at 3:52 PM Jann Horn <jannh@google.com> wrote: > > > Do we still need this patch? > > I just tried removing this patch from the series; without it, the > kmem_cache_invalid_free kunit test fails because the kmem_cache_free() > no longer synchronously notices that the pointer is misaligned. I > guess I could change the testcase like this to make the tests pass > without this patch, but I'd like to hear from you or another KASAN > person whether you think that's a reasonable change: Ah, I see. I think detecting a bug earlier if we can is better. So I don't mind keeping this patch, was just confused by the commit message. Adding on top of my comments from before: I think if you move check_slab_free() out of poison_slab_object() (but add to __kasan_mempool_poison_object()), and move is_kfence_address() and kasan_arch_is_ready() to poison_slab_object()'s callers, you won't even need the free_validation_result enum, so the patch should become simpler. You can also rename check_slab_free() to check_slab_allocation() to make it be named similarly to the already existing check_page_allocation(). (I think we should also later move kasan_arch_is_ready() out of check_page_allocation() into the high-level hooks for consistency; it also seems cleaner to have all of these ignore checks in the high-level functions instead of lower-level inlined ones.) Thanks!
On Fri, Jul 26, 2024 at 2:43 AM Andrey Konovalov <andreyknvl@gmail.com> wrote: > On Thu, Jul 25, 2024 at 5:32 PM Jann Horn <jannh@google.com> wrote: > > Currently, when KASAN is combined with init-on-free behavior, the > > initialization happens before KASAN's "invalid free" checks. [...] > > So add a new KASAN hook that allows KASAN to pre-validate a > > kmem_cache_free() operation before SLUB actually starts modifying the > > object or its metadata. > > > > Acked-by: Vlastimil Babka <vbabka@suse.cz> #slub > > Signed-off-by: Jann Horn <jannh@google.com> > > --- > > include/linux/kasan.h | 16 ++++++++++++++++ > > mm/kasan/common.c | 51 +++++++++++++++++++++++++++++++++++++++------------ > > mm/slub.c | 7 +++++++ > > 3 files changed, 62 insertions(+), 12 deletions(-) > > > > diff --git a/include/linux/kasan.h b/include/linux/kasan.h > > index 70d6a8f6e25d..ebd93c843e78 100644 > > --- a/include/linux/kasan.h > > +++ b/include/linux/kasan.h > > @@ -175,6 +175,16 @@ static __always_inline void * __must_check kasan_init_slab_obj( > > return (void *)object; > > } > > > > +bool __kasan_slab_pre_free(struct kmem_cache *s, void *object, > > + unsigned long ip); > > +static __always_inline bool kasan_slab_pre_free(struct kmem_cache *s, > > + void *object) > > +{ > > + if (kasan_enabled()) > > + return __kasan_slab_pre_free(s, object, _RET_IP_); > > + return false; > > +} > > Please add a documentation comment for this new hook; something like > what we have for kasan_mempool_poison_pages() and some of the others. > (I've been meaning to add them for all of them, but still didn't get > around to that.) Ack, done in v4. > > +static inline bool poison_slab_object(struct kmem_cache *cache, void *object, > > + unsigned long ip, bool init) > > +{ > > + void *tagged_object = object; > > + enum free_validation_result valid = check_slab_free(cache, object, ip); > > I believe we don't need check_slab_free() here, as it was already done > in kasan_slab_pre_free()? Checking just kasan_arch_is_ready() and > is_kfence_address() should save a bit on performance impact. > > Though if we remove check_slab_free() from here, we do need to add it > to __kasan_mempool_poison_object(). Ack, changed in v4. > > + > > + if (valid == KASAN_FREE_IS_IGNORED) > > + return false; > > + if (valid == KASAN_FREE_IS_INVALID) > > + return true; > > + > > + 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)) > > + return false; > > I vaguely recall there was some reason why this check was done before > the kasan_byte_accessible() check, but I might be wrong. Could you try > booting the kernel with only this patch applied to see if anything > breaks? I tried booting it to a graphical environment and running the kunit tests, nothing immediately broke from what I can tell...
On Sat, Jul 27, 2024 at 2:47 AM Andrey Konovalov <andreyknvl@gmail.com> wrote: > On Fri, Jul 26, 2024 at 3:52 PM Jann Horn <jannh@google.com> wrote: > > > > > Do we still need this patch? > > > > I just tried removing this patch from the series; without it, the > > kmem_cache_invalid_free kunit test fails because the kmem_cache_free() > > no longer synchronously notices that the pointer is misaligned. I > > guess I could change the testcase like this to make the tests pass > > without this patch, but I'd like to hear from you or another KASAN > > person whether you think that's a reasonable change: > > Ah, I see. I think detecting a bug earlier if we can is better. So I > don't mind keeping this patch, was just confused by the commit > message. ack, changed it in v4 > Adding on top of my comments from before: I think if you move > check_slab_free() out of poison_slab_object() (but add to > __kasan_mempool_poison_object()), and move is_kfence_address() and > kasan_arch_is_ready() to poison_slab_object()'s callers, you won't > even need the free_validation_result enum, so the patch should become > simpler. right, makes sense, changed in v4 > You can also rename check_slab_free() to check_slab_allocation() to > make it be named similarly to the already existing > check_page_allocation(). done in v4
diff --git a/include/linux/kasan.h b/include/linux/kasan.h index 70d6a8f6e25d..ebd93c843e78 100644 --- a/include/linux/kasan.h +++ b/include/linux/kasan.h @@ -175,6 +175,16 @@ static __always_inline void * __must_check kasan_init_slab_obj( return (void *)object; } +bool __kasan_slab_pre_free(struct kmem_cache *s, void *object, + unsigned long ip); +static __always_inline bool kasan_slab_pre_free(struct kmem_cache *s, + void *object) +{ + if (kasan_enabled()) + return __kasan_slab_pre_free(s, object, _RET_IP_); + return false; +} + bool __kasan_slab_free(struct kmem_cache *s, void *object, unsigned long ip, bool init); static __always_inline bool kasan_slab_free(struct kmem_cache *s, @@ -371,6 +381,12 @@ static inline void *kasan_init_slab_obj(struct kmem_cache *cache, { return (void *)object; } + +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) { return false; diff --git a/mm/kasan/common.c b/mm/kasan/common.c index 85e7c6b4575c..7c7fc6ce7eb7 100644 --- a/mm/kasan/common.c +++ b/mm/kasan/common.c @@ -208,31 +208,52 @@ void * __must_check __kasan_init_slab_obj(struct kmem_cache *cache, return (void *)object; } -static inline bool poison_slab_object(struct kmem_cache *cache, void *object, - unsigned long ip, bool init) +enum free_validation_result { + KASAN_FREE_IS_IGNORED, + KASAN_FREE_IS_VALID, + KASAN_FREE_IS_INVALID +}; + +static enum free_validation_result check_slab_free(struct kmem_cache *cache, + void *object, unsigned long ip) { - void *tagged_object; + void *tagged_object = object; - if (!kasan_arch_is_ready()) - return false; + if (is_kfence_address(object) || !kasan_arch_is_ready()) + return KASAN_FREE_IS_IGNORED; - tagged_object = object; object = kasan_reset_tag(object); if (unlikely(nearest_obj(cache, virt_to_slab(object), object) != object)) { kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_INVALID_FREE); - return true; + return KASAN_FREE_IS_INVALID; } - /* RCU slabs could be legally used after free within the RCU period. */ - if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU)) - return false; - if (!kasan_byte_accessible(tagged_object)) { kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_DOUBLE_FREE); - return true; + return KASAN_FREE_IS_INVALID; } + return KASAN_FREE_IS_VALID; +} + +static inline bool poison_slab_object(struct kmem_cache *cache, void *object, + unsigned long ip, bool init) +{ + void *tagged_object = object; + enum free_validation_result valid = check_slab_free(cache, object, ip); + + if (valid == KASAN_FREE_IS_IGNORED) + return false; + if (valid == KASAN_FREE_IS_INVALID) + return true; + + 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)) + return false; + kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE), KASAN_SLAB_FREE, init); @@ -242,6 +263,12 @@ static inline bool poison_slab_object(struct kmem_cache *cache, void *object, return false; } +bool __kasan_slab_pre_free(struct kmem_cache *cache, void *object, + unsigned long ip) +{ + return check_slab_free(cache, object, ip) == KASAN_FREE_IS_INVALID; +} + bool __kasan_slab_free(struct kmem_cache *cache, void *object, unsigned long ip, bool init) { diff --git a/mm/slub.c b/mm/slub.c index 4927edec6a8c..34724704c52d 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -2170,6 +2170,13 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init) 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; + /* * As memory initialization might be integrated into KASAN, * kasan_slab_free and initialization memset's must be