mm/kasan/common.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-)
Currently, enabling KASAN masks bugs where a lockless lookup path gets a
pointer to a SLAB_TYPESAFE_BY_RCU object that might concurrently be
recycled and is insufficiently careful about handling recycled objects:
KASAN puts freed objects in SLAB_TYPESAFE_BY_RCU slabs onto its quarantine
queues, even when it can't actually detect UAF in these objects, and the
quarantine prevents fast recycling.
When I introduced CONFIG_SLUB_RCU_DEBUG, my intention was that enabling
CONFIG_SLUB_RCU_DEBUG should cause KASAN to mark such objects as freed
after an RCU grace period and put them on the quarantine, while disabling
CONFIG_SLUB_RCU_DEBUG should allow such objects to be reused immediately;
but that hasn't actually been working.
I discovered such a UAF bug involving SLAB_TYPESAFE_BY_RCU yesterday; I
could only trigger this bug in a KASAN build by disabling
CONFIG_SLUB_RCU_DEBUG and applying this patch.
Signed-off-by: Jann Horn <jannh@google.com>
---
mm/kasan/common.c | 25 ++++++++++++++++++-------
1 file changed, 18 insertions(+), 7 deletions(-)
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index ed4873e18c75..9142964ab9c9 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -230,16 +230,12 @@ static bool check_slab_allocation(struct kmem_cache *cache, void *object,
}
static inline void poison_slab_object(struct kmem_cache *cache, void *object,
- bool init, bool still_accessible)
+ bool init)
{
void *tagged_object = object;
object = kasan_reset_tag(object);
- /* RCU slabs could be legally used after free within the RCU period. */
- if (unlikely(still_accessible))
- return;
-
kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
KASAN_SLAB_FREE, init);
@@ -261,7 +257,22 @@ bool __kasan_slab_free(struct kmem_cache *cache, void *object, bool init,
if (!kasan_arch_is_ready() || is_kfence_address(object))
return false;
- poison_slab_object(cache, object, init, still_accessible);
+ /*
+ * If this point is reached with an object that must still be
+ * accessible under RCU, we can't poison it; in that case, also skip the
+ * quarantine. This should mostly only happen when CONFIG_SLUB_RCU_DEBUG
+ * has been disabled manually.
+ *
+ * Putting the object on the quarantine wouldn't help catch UAFs (since
+ * we can't poison it here), and it would mask bugs caused by
+ * SLAB_TYPESAFE_BY_RCU users not being careful enough about object
+ * reuse; so overall, putting the object into the quarantine here would
+ * be counterproductive.
+ */
+ if (still_accessible)
+ return false;
+
+ poison_slab_object(cache, object, init);
/*
* If the object is put into quarantine, do not let slab put the object
@@ -519,7 +530,7 @@ bool __kasan_mempool_poison_object(void *ptr, unsigned long ip)
if (check_slab_allocation(slab->slab_cache, ptr, ip))
return false;
- poison_slab_object(slab->slab_cache, ptr, false, false);
+ poison_slab_object(slab->slab_cache, ptr, false);
return true;
}
---
base-commit: 89be9a83ccf1f88522317ce02f854f30d6115c41
change-id: 20250723-kasan-tsbrcu-noquarantine-e207bb990e24
--
Jann Horn <jannh@google.com>On Wed, Jul 23, 2025 at 4:59 PM Jann Horn <jannh@google.com> wrote:
>
> Currently, enabling KASAN masks bugs where a lockless lookup path gets a
> pointer to a SLAB_TYPESAFE_BY_RCU object that might concurrently be
> recycled and is insufficiently careful about handling recycled objects:
> KASAN puts freed objects in SLAB_TYPESAFE_BY_RCU slabs onto its quarantine
> queues, even when it can't actually detect UAF in these objects, and the
> quarantine prevents fast recycling.
>
> When I introduced CONFIG_SLUB_RCU_DEBUG, my intention was that enabling
> CONFIG_SLUB_RCU_DEBUG should cause KASAN to mark such objects as freed
> after an RCU grace period and put them on the quarantine, while disabling
> CONFIG_SLUB_RCU_DEBUG should allow such objects to be reused immediately;
> but that hasn't actually been working.
>
> I discovered such a UAF bug involving SLAB_TYPESAFE_BY_RCU yesterday; I
> could only trigger this bug in a KASAN build by disabling
> CONFIG_SLUB_RCU_DEBUG and applying this patch.
>
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
> mm/kasan/common.c | 25 ++++++++++++++++++-------
> 1 file changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index ed4873e18c75..9142964ab9c9 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -230,16 +230,12 @@ static bool check_slab_allocation(struct kmem_cache *cache, void *object,
> }
>
> static inline void poison_slab_object(struct kmem_cache *cache, void *object,
> - bool init, bool still_accessible)
> + bool init)
> {
> void *tagged_object = object;
>
> object = kasan_reset_tag(object);
>
> - /* RCU slabs could be legally used after free within the RCU period. */
> - if (unlikely(still_accessible))
> - return;
> -
> kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
> KASAN_SLAB_FREE, init);
>
> @@ -261,7 +257,22 @@ bool __kasan_slab_free(struct kmem_cache *cache, void *object, bool init,
> if (!kasan_arch_is_ready() || is_kfence_address(object))
> return false;
>
> - poison_slab_object(cache, object, init, still_accessible);
> + /*
> + * If this point is reached with an object that must still be
> + * accessible under RCU, we can't poison it; in that case, also skip the
> + * quarantine. This should mostly only happen when CONFIG_SLUB_RCU_DEBUG
> + * has been disabled manually.
> + *
> + * Putting the object on the quarantine wouldn't help catch UAFs (since
> + * we can't poison it here), and it would mask bugs caused by
> + * SLAB_TYPESAFE_BY_RCU users not being careful enough about object
> + * reuse; so overall, putting the object into the quarantine here would
> + * be counterproductive.
> + */
> + if (still_accessible)
> + return false;
> +
> + poison_slab_object(cache, object, init);
>
> /*
> * If the object is put into quarantine, do not let slab put the object
> @@ -519,7 +530,7 @@ bool __kasan_mempool_poison_object(void *ptr, unsigned long ip)
> if (check_slab_allocation(slab->slab_cache, ptr, ip))
> return false;
>
> - poison_slab_object(slab->slab_cache, ptr, false, false);
> + poison_slab_object(slab->slab_cache, ptr, false);
> return true;
> }
>
>
> ---
> base-commit: 89be9a83ccf1f88522317ce02f854f30d6115c41
> change-id: 20250723-kasan-tsbrcu-noquarantine-e207bb990e24
>
> --
> Jann Horn <jannh@google.com>
>
Acked-by: Andrey Konovalov <andreyknvl@gmail.com>
Would it be hard to add KUnit test to check that KASAN detects such issues?
On Sun, Jul 27, 2025 at 12:06 AM Andrey Konovalov <andreyknvl@gmail.com> wrote:
> On Wed, Jul 23, 2025 at 4:59 PM Jann Horn <jannh@google.com> wrote:
> >
> > Currently, enabling KASAN masks bugs where a lockless lookup path gets a
> > pointer to a SLAB_TYPESAFE_BY_RCU object that might concurrently be
> > recycled and is insufficiently careful about handling recycled objects:
> > KASAN puts freed objects in SLAB_TYPESAFE_BY_RCU slabs onto its quarantine
> > queues, even when it can't actually detect UAF in these objects, and the
> > quarantine prevents fast recycling.
> >
> > When I introduced CONFIG_SLUB_RCU_DEBUG, my intention was that enabling
> > CONFIG_SLUB_RCU_DEBUG should cause KASAN to mark such objects as freed
> > after an RCU grace period and put them on the quarantine, while disabling
> > CONFIG_SLUB_RCU_DEBUG should allow such objects to be reused immediately;
> > but that hasn't actually been working.
> >
> > I discovered such a UAF bug involving SLAB_TYPESAFE_BY_RCU yesterday; I
> > could only trigger this bug in a KASAN build by disabling
> > CONFIG_SLUB_RCU_DEBUG and applying this patch.
> >
> > Signed-off-by: Jann Horn <jannh@google.com>
> > ---
> > mm/kasan/common.c | 25 ++++++++++++++++++-------
> > 1 file changed, 18 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> > index ed4873e18c75..9142964ab9c9 100644
> > --- a/mm/kasan/common.c
> > +++ b/mm/kasan/common.c
> > @@ -230,16 +230,12 @@ static bool check_slab_allocation(struct kmem_cache *cache, void *object,
> > }
> >
> > static inline void poison_slab_object(struct kmem_cache *cache, void *object,
> > - bool init, bool still_accessible)
> > + bool init)
> > {
> > void *tagged_object = object;
> >
> > object = kasan_reset_tag(object);
> >
> > - /* RCU slabs could be legally used after free within the RCU period. */
> > - if (unlikely(still_accessible))
> > - return;
> > -
> > kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
> > KASAN_SLAB_FREE, init);
> >
> > @@ -261,7 +257,22 @@ bool __kasan_slab_free(struct kmem_cache *cache, void *object, bool init,
> > if (!kasan_arch_is_ready() || is_kfence_address(object))
> > return false;
> >
> > - poison_slab_object(cache, object, init, still_accessible);
> > + /*
> > + * If this point is reached with an object that must still be
> > + * accessible under RCU, we can't poison it; in that case, also skip the
> > + * quarantine. This should mostly only happen when CONFIG_SLUB_RCU_DEBUG
> > + * has been disabled manually.
> > + *
> > + * Putting the object on the quarantine wouldn't help catch UAFs (since
> > + * we can't poison it here), and it would mask bugs caused by
> > + * SLAB_TYPESAFE_BY_RCU users not being careful enough about object
> > + * reuse; so overall, putting the object into the quarantine here would
> > + * be counterproductive.
> > + */
> > + if (still_accessible)
> > + return false;
> > +
> > + poison_slab_object(cache, object, init);
> >
> > /*
> > * If the object is put into quarantine, do not let slab put the object
> > @@ -519,7 +530,7 @@ bool __kasan_mempool_poison_object(void *ptr, unsigned long ip)
> > if (check_slab_allocation(slab->slab_cache, ptr, ip))
> > return false;
> >
> > - poison_slab_object(slab->slab_cache, ptr, false, false);
> > + poison_slab_object(slab->slab_cache, ptr, false);
> > return true;
> > }
> >
> >
> > ---
> > base-commit: 89be9a83ccf1f88522317ce02f854f30d6115c41
> > change-id: 20250723-kasan-tsbrcu-noquarantine-e207bb990e24
> >
> > --
> > Jann Horn <jannh@google.com>
> >
>
> Acked-by: Andrey Konovalov <andreyknvl@gmail.com>
Thanks!
> Would it be hard to add KUnit test to check that KASAN detects such issues?
Sent a separate patch with a kunit test.
On 7/23/25 16:59, Jann Horn wrote:
> Currently, enabling KASAN masks bugs where a lockless lookup path gets a
> pointer to a SLAB_TYPESAFE_BY_RCU object that might concurrently be
> recycled and is insufficiently careful about handling recycled objects:
> KASAN puts freed objects in SLAB_TYPESAFE_BY_RCU slabs onto its quarantine
> queues, even when it can't actually detect UAF in these objects, and the
> quarantine prevents fast recycling.
>
> When I introduced CONFIG_SLUB_RCU_DEBUG, my intention was that enabling
> CONFIG_SLUB_RCU_DEBUG should cause KASAN to mark such objects as freed
> after an RCU grace period and put them on the quarantine, while disabling
> CONFIG_SLUB_RCU_DEBUG should allow such objects to be reused immediately;
> but that hasn't actually been working.
Was the "allow reuse immediately" not working also before you introduced
CONFIG_SLUB_RCU_DEBUG, or is it a side-effect of that? IOW should we add a
Fixes: here?
> I discovered such a UAF bug involving SLAB_TYPESAFE_BY_RCU yesterday; I
> could only trigger this bug in a KASAN build by disabling
> CONFIG_SLUB_RCU_DEBUG and applying this patch.
>
> Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> mm/kasan/common.c | 25 ++++++++++++++++++-------
> 1 file changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index ed4873e18c75..9142964ab9c9 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -230,16 +230,12 @@ static bool check_slab_allocation(struct kmem_cache *cache, void *object,
> }
>
> static inline void poison_slab_object(struct kmem_cache *cache, void *object,
> - bool init, bool still_accessible)
> + bool init)
> {
> void *tagged_object = object;
>
> object = kasan_reset_tag(object);
>
> - /* RCU slabs could be legally used after free within the RCU period. */
> - if (unlikely(still_accessible))
> - return;
> -
> kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
> KASAN_SLAB_FREE, init);
>
> @@ -261,7 +257,22 @@ bool __kasan_slab_free(struct kmem_cache *cache, void *object, bool init,
> if (!kasan_arch_is_ready() || is_kfence_address(object))
> return false;
>
> - poison_slab_object(cache, object, init, still_accessible);
> + /*
> + * If this point is reached with an object that must still be
> + * accessible under RCU, we can't poison it; in that case, also skip the
> + * quarantine. This should mostly only happen when CONFIG_SLUB_RCU_DEBUG
> + * has been disabled manually.
> + *
> + * Putting the object on the quarantine wouldn't help catch UAFs (since
> + * we can't poison it here), and it would mask bugs caused by
> + * SLAB_TYPESAFE_BY_RCU users not being careful enough about object
> + * reuse; so overall, putting the object into the quarantine here would
> + * be counterproductive.
> + */
> + if (still_accessible)
> + return false;
> +
> + poison_slab_object(cache, object, init);
>
> /*
> * If the object is put into quarantine, do not let slab put the object
> @@ -519,7 +530,7 @@ bool __kasan_mempool_poison_object(void *ptr, unsigned long ip)
> if (check_slab_allocation(slab->slab_cache, ptr, ip))
> return false;
>
> - poison_slab_object(slab->slab_cache, ptr, false, false);
> + poison_slab_object(slab->slab_cache, ptr, false);
> return true;
> }
>
>
> ---
> base-commit: 89be9a83ccf1f88522317ce02f854f30d6115c41
> change-id: 20250723-kasan-tsbrcu-noquarantine-e207bb990e24
>
On Thu, Jul 24, 2025 at 12:14 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> On 7/23/25 16:59, Jann Horn wrote:
> > Currently, enabling KASAN masks bugs where a lockless lookup path gets a
> > pointer to a SLAB_TYPESAFE_BY_RCU object that might concurrently be
> > recycled and is insufficiently careful about handling recycled objects:
> > KASAN puts freed objects in SLAB_TYPESAFE_BY_RCU slabs onto its quarantine
> > queues, even when it can't actually detect UAF in these objects, and the
> > quarantine prevents fast recycling.
> >
> > When I introduced CONFIG_SLUB_RCU_DEBUG, my intention was that enabling
> > CONFIG_SLUB_RCU_DEBUG should cause KASAN to mark such objects as freed
> > after an RCU grace period and put them on the quarantine, while disabling
> > CONFIG_SLUB_RCU_DEBUG should allow such objects to be reused immediately;
> > but that hasn't actually been working.
>
> Was the "allow reuse immediately" not working also before you introduced
> CONFIG_SLUB_RCU_DEBUG, or is it a side-effect of that? IOW should we add a
> Fixes: here?
This was already an issue before. I think it got broken by refactoring
in commit b556a462eb8df6b6836c318d23f43409c40a7c7e ("kasan: save free
stack traces for slab mempools"), but I don't think it was necessarily
an intentionally supported feature.
> > I discovered such a UAF bug involving SLAB_TYPESAFE_BY_RCU yesterday; I
> > could only trigger this bug in a KASAN build by disabling
> > CONFIG_SLUB_RCU_DEBUG and applying this patch.
> >
> > Signed-off-by: Jann Horn <jannh@google.com>
>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
Thanks!
On Thu, Jul 24, 2025 at 12:14 PM Vlastimil Babka <vbabka@suse.cz> wrote: > > On 7/23/25 16:59, Jann Horn wrote: > > Currently, enabling KASAN masks bugs where a lockless lookup path gets a > > pointer to a SLAB_TYPESAFE_BY_RCU object that might concurrently be > > recycled and is insufficiently careful about handling recycled objects: > > KASAN puts freed objects in SLAB_TYPESAFE_BY_RCU slabs onto its quarantine > > queues, even when it can't actually detect UAF in these objects, and the > > quarantine prevents fast recycling. > > > > When I introduced CONFIG_SLUB_RCU_DEBUG, my intention was that enabling > > CONFIG_SLUB_RCU_DEBUG should cause KASAN to mark such objects as freed > > after an RCU grace period and put them on the quarantine, while disabling > > CONFIG_SLUB_RCU_DEBUG should allow such objects to be reused immediately; > > but that hasn't actually been working. > > Was the "allow reuse immediately" not working also before you introduced > CONFIG_SLUB_RCU_DEBUG, or is it a side-effect of that? IOW should we add a > Fixes: here? > > > I discovered such a UAF bug involving SLAB_TYPESAFE_BY_RCU yesterday; I > > could only trigger this bug in a KASAN build by disabling > > CONFIG_SLUB_RCU_DEBUG and applying this patch. > > > > Signed-off-by: Jann Horn <jannh@google.com> > > Acked-by: Vlastimil Babka <vbabka@suse.cz> Reviewed-by: Alexander Potapenko <glider@google.com>
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