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.)
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;
+}
+
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
--
2.45.2.1089.g2a221341d9-googOn 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.
[...]
> > 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 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 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
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