mm/Kconfig | 1 + 1 file changed, 1 insertion(+)
Disable CONFIG_KVFREE_RCU_BATCHED in CONFIG_RCU_STRICT_GRACE_PERIOD builds
so that kernel fuzzers have an easier time finding use-after-free involving
kfree_rcu().
The intent behind CONFIG_RCU_STRICT_GRACE_PERIOD is that RCU should invoke
callbacks and free objects as soon as possible (at a large performance
cost) so that kernel fuzzers and such have an easier time detecting
use-after-free bugs in objects with RCU lifetime.
CONFIG_KVFREE_RCU_BATCHED is a performance optimization that queues
RCU-freed objects in ways that CONFIG_RCU_STRICT_GRACE_PERIOD can't
expedite; for example, the following testcase doesn't trigger a KASAN splat
when CONFIG_KVFREE_RCU_BATCHED is enabled:
```
struct foo_struct {
struct rcu_head rcu;
int a;
};
struct foo_struct *foo = kmalloc(sizeof(*foo),
GFP_KERNEL | __GFP_NOFAIL | __GFP_ZERO);
pr_info("%s: calling kfree_rcu()\n", __func__);
kfree_rcu(foo, rcu);
msleep(10);
pr_info("%s: start UAF access\n", __func__);
READ_ONCE(foo->a);
pr_info("%s: end UAF access\n", __func__);
```
Signed-off-by: Jann Horn <jannh@google.com>
---
mm/Kconfig | 1 +
1 file changed, 1 insertion(+)
diff --git a/mm/Kconfig b/mm/Kconfig
index ebd8ea353687..67a72fe89186 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -172,6 +172,7 @@ config SLUB
config KVFREE_RCU_BATCHED
def_bool y
depends on !SLUB_TINY && !TINY_RCU
+ depends on !RCU_STRICT_GRACE_PERIOD
config SLUB_TINY
bool "Configure for minimal memory footprint"
---
base-commit: b29fb8829bff243512bb8c8908fd39406f9fd4c3
change-id: 20260324-kasan-kfree-rcu-4e7f490237ef
--
Jann Horn <jannh@google.com>On 3/24/26 22:35, Jann Horn wrote:
> Disable CONFIG_KVFREE_RCU_BATCHED in CONFIG_RCU_STRICT_GRACE_PERIOD builds
> so that kernel fuzzers have an easier time finding use-after-free involving
> kfree_rcu().
>
> The intent behind CONFIG_RCU_STRICT_GRACE_PERIOD is that RCU should invoke
> callbacks and free objects as soon as possible (at a large performance
> cost) so that kernel fuzzers and such have an easier time detecting
> use-after-free bugs in objects with RCU lifetime.
>
> CONFIG_KVFREE_RCU_BATCHED is a performance optimization that queues
> RCU-freed objects in ways that CONFIG_RCU_STRICT_GRACE_PERIOD can't
> expedite; for example, the following testcase doesn't trigger a KASAN splat
> when CONFIG_KVFREE_RCU_BATCHED is enabled:
> ```
> struct foo_struct {
> struct rcu_head rcu;
> int a;
> };
> struct foo_struct *foo = kmalloc(sizeof(*foo),
> GFP_KERNEL | __GFP_NOFAIL | __GFP_ZERO);
>
> pr_info("%s: calling kfree_rcu()\n", __func__);
> kfree_rcu(foo, rcu);
> msleep(10);
> pr_info("%s: start UAF access\n", __func__);
> READ_ONCE(foo->a);
> pr_info("%s: end UAF access\n", __func__);
> ```
>
> Signed-off-by: Jann Horn <jannh@google.com>
Hm but with 7.0 we have sheaves everywhere including kmalloc caches, and
there's a percpu rcu_free sheaf collecting kfree_rcu'd objects. Only when
it's full it's submitted to call_rcu() where the callback rcu_free_sheaf()
runs slab_free_hook() including kasan hooks etc. If there's nothing filling
the rcu_free sheaf, the objects can sit there possibly indefinitely.
That means CONFIG_KVFREE_RCU_BATCHED now handles only the rare cases where
kfree_rcu() to the rcu_free sheaf fails (and I still owe it to Ulad to do
something about this).
So to complete the intent of this patch, we should perhaps also skip the
rcu_free sheaf with RCU_STRICT_GRACE_PERIOD? (or with !KVFREE_RCU_BATCHED
perhaps as it's also a form of batching).
But then I wonder if the testcase in the changelog appeared to be fixed with
this patch on a 7.0-rcX kernel (base-commit: below is rc3+) because by my
understanding it shouldn't have been. (unless there happened to be enough
kfree_rcu() activity on that cpu+kmalloc cache combination, so that the
rcu_free sheaf got submitted withing that msleep(10)).
> ---
> mm/Kconfig | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index ebd8ea353687..67a72fe89186 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -172,6 +172,7 @@ config SLUB
> config KVFREE_RCU_BATCHED
> def_bool y
> depends on !SLUB_TINY && !TINY_RCU
> + depends on !RCU_STRICT_GRACE_PERIOD
>
> config SLUB_TINY
> bool "Configure for minimal memory footprint"
>
> ---
> base-commit: b29fb8829bff243512bb8c8908fd39406f9fd4c3
> change-id: 20260324-kasan-kfree-rcu-4e7f490237ef
>
> --
> Jann Horn <jannh@google.com>
>
On Wed, Mar 25, 2026 at 08:50:07AM +0100, Vlastimil Babka (SUSE) wrote:
> On 3/24/26 22:35, Jann Horn wrote:
> > Disable CONFIG_KVFREE_RCU_BATCHED in CONFIG_RCU_STRICT_GRACE_PERIOD builds
> > so that kernel fuzzers have an easier time finding use-after-free involving
> > kfree_rcu().
> >
> > The intent behind CONFIG_RCU_STRICT_GRACE_PERIOD is that RCU should invoke
> > callbacks and free objects as soon as possible (at a large performance
> > cost) so that kernel fuzzers and such have an easier time detecting
> > use-after-free bugs in objects with RCU lifetime.
> >
> > CONFIG_KVFREE_RCU_BATCHED is a performance optimization that queues
> > RCU-freed objects in ways that CONFIG_RCU_STRICT_GRACE_PERIOD can't
> > expedite; for example, the following testcase doesn't trigger a KASAN splat
> > when CONFIG_KVFREE_RCU_BATCHED is enabled:
> > ```
> > struct foo_struct {
> > struct rcu_head rcu;
> > int a;
> > };
> > struct foo_struct *foo = kmalloc(sizeof(*foo),
> > GFP_KERNEL | __GFP_NOFAIL | __GFP_ZERO);
> >
> > pr_info("%s: calling kfree_rcu()\n", __func__);
> > kfree_rcu(foo, rcu);
> > msleep(10);
> > pr_info("%s: start UAF access\n", __func__);
> > READ_ONCE(foo->a);
> > pr_info("%s: end UAF access\n", __func__);
> > ```
> >
> > Signed-off-by: Jann Horn <jannh@google.com>
>
> Hm but with 7.0 we have sheaves everywhere including kmalloc caches, and
> there's a percpu rcu_free sheaf collecting kfree_rcu'd objects.
Right, but only when CONFIG_KVFREE_RCU_BATCHED=y
> Only when
> it's full it's submitted to call_rcu() where the callback rcu_free_sheaf()
> runs slab_free_hook() including kasan hooks etc. If there's nothing filling
> the rcu_free sheaf, the objects can sit there possibly indefinitely.
Right.
> That means CONFIG_KVFREE_RCU_BATCHED now handles only the rare cases where
> kfree_rcu() to the rcu_free sheaf fails (and I still owe it to Ulad to do
> something about this).
Right.
> So to complete the intent of this patch, we should perhaps also skip the
> rcu_free sheaf with RCU_STRICT_GRACE_PERIOD? (or with !KVFREE_RCU_BATCHED
> perhaps as it's also a form of batching).
Maybe I'm missing something, but...
by making KVFREE_RCU_BATCHED depend on !RCU_STRICT_GRACE_PERIOD,
selecting RCU_STRICT_GRACE_PERIOD disables all uses of rcu_free sheaves?
kvfree_call_rcu() implementation on !KVFREE_RCU_BATCHED does not call
kfree_rcu_sheaf().
> But then I wonder if the testcase in the changelog appeared to be fixed with
> this patch on a 7.0-rcX kernel (base-commit: below is rc3+) because by my
> understanding it shouldn't have been. (unless there happened to be enough
> kfree_rcu() activity on that cpu+kmalloc cache combination, so that the
> rcu_free sheaf got submitted withing that msleep(10)).
>
> > ---
> > mm/Kconfig | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index ebd8ea353687..67a72fe89186 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -172,6 +172,7 @@ config SLUB
> > config KVFREE_RCU_BATCHED
> > def_bool y
> > depends on !SLUB_TINY && !TINY_RCU
> > + depends on !RCU_STRICT_GRACE_PERIOD
> >
> > config SLUB_TINY
> > bool "Configure for minimal memory footprint"
> >
> > ---
> > base-commit: b29fb8829bff243512bb8c8908fd39406f9fd4c3
> > change-id: 20260324-kasan-kfree-rcu-4e7f490237ef
> >
> > --
> > Jann Horn <jannh@google.com>
> >
>
>
--
Cheers,
Harry / Hyeonggon
On 3/25/26 09:21, Harry Yoo (Oracle) wrote:
> On Wed, Mar 25, 2026 at 08:50:07AM +0100, Vlastimil Babka (SUSE) wrote:
>> On 3/24/26 22:35, Jann Horn wrote:
>> > Disable CONFIG_KVFREE_RCU_BATCHED in CONFIG_RCU_STRICT_GRACE_PERIOD builds
>> > so that kernel fuzzers have an easier time finding use-after-free involving
>> > kfree_rcu().
>> >
>> > The intent behind CONFIG_RCU_STRICT_GRACE_PERIOD is that RCU should invoke
>> > callbacks and free objects as soon as possible (at a large performance
>> > cost) so that kernel fuzzers and such have an easier time detecting
>> > use-after-free bugs in objects with RCU lifetime.
>> >
>> > CONFIG_KVFREE_RCU_BATCHED is a performance optimization that queues
>> > RCU-freed objects in ways that CONFIG_RCU_STRICT_GRACE_PERIOD can't
>> > expedite; for example, the following testcase doesn't trigger a KASAN splat
>> > when CONFIG_KVFREE_RCU_BATCHED is enabled:
>> > ```
>> > struct foo_struct {
>> > struct rcu_head rcu;
>> > int a;
>> > };
>> > struct foo_struct *foo = kmalloc(sizeof(*foo),
>> > GFP_KERNEL | __GFP_NOFAIL | __GFP_ZERO);
>> >
>> > pr_info("%s: calling kfree_rcu()\n", __func__);
>> > kfree_rcu(foo, rcu);
>> > msleep(10);
>> > pr_info("%s: start UAF access\n", __func__);
>> > READ_ONCE(foo->a);
>> > pr_info("%s: end UAF access\n", __func__);
>> > ```
>> >
>> > Signed-off-by: Jann Horn <jannh@google.com>
>>
>> Hm but with 7.0 we have sheaves everywhere including kmalloc caches, and
>> there's a percpu rcu_free sheaf collecting kfree_rcu'd objects.
>
> Right, but only when CONFIG_KVFREE_RCU_BATCHED=y
>
>> Only when
>> it's full it's submitted to call_rcu() where the callback rcu_free_sheaf()
>> runs slab_free_hook() including kasan hooks etc. If there's nothing filling
>> the rcu_free sheaf, the objects can sit there possibly indefinitely.
>
> Right.
>
>> That means CONFIG_KVFREE_RCU_BATCHED now handles only the rare cases where
>> kfree_rcu() to the rcu_free sheaf fails (and I still owe it to Ulad to do
>> something about this).
>
> Right.
>
>> So to complete the intent of this patch, we should perhaps also skip the
>> rcu_free sheaf with RCU_STRICT_GRACE_PERIOD? (or with !KVFREE_RCU_BATCHED
>> perhaps as it's also a form of batching).
>
> Maybe I'm missing something, but...
>
> by making KVFREE_RCU_BATCHED depend on !RCU_STRICT_GRACE_PERIOD,
> selecting RCU_STRICT_GRACE_PERIOD disables all uses of rcu_free sheaves?
>
> kvfree_call_rcu() implementation on !KVFREE_RCU_BATCHED does not call
> kfree_rcu_sheaf().
Ah yeah, I missed that there are two kvfree_call_rcu() implementations and
kfree_rcu_sheaf() is only used in the batched one. Sorry for the noise.
Will queue the patch
>> But then I wonder if the testcase in the changelog appeared to be fixed with
>> this patch on a 7.0-rcX kernel (base-commit: below is rc3+) because by my
>> understanding it shouldn't have been. (unless there happened to be enough
>> kfree_rcu() activity on that cpu+kmalloc cache combination, so that the
>> rcu_free sheaf got submitted withing that msleep(10)).
>>
>> > ---
>> > mm/Kconfig | 1 +
>> > 1 file changed, 1 insertion(+)
>> >
>> > diff --git a/mm/Kconfig b/mm/Kconfig
>> > index ebd8ea353687..67a72fe89186 100644
>> > --- a/mm/Kconfig
>> > +++ b/mm/Kconfig
>> > @@ -172,6 +172,7 @@ config SLUB
>> > config KVFREE_RCU_BATCHED
>> > def_bool y
>> > depends on !SLUB_TINY && !TINY_RCU
>> > + depends on !RCU_STRICT_GRACE_PERIOD
>> >
>> > config SLUB_TINY
>> > bool "Configure for minimal memory footprint"
>> >
>> > ---
>> > base-commit: b29fb8829bff243512bb8c8908fd39406f9fd4c3
>> > change-id: 20260324-kasan-kfree-rcu-4e7f490237ef
>> >
>> > --
>> > Jann Horn <jannh@google.com>
>> >
>>
>>
>
On Wed, Mar 25, 2026 at 09:34:40AM +0100, Vlastimil Babka (SUSE) wrote:
> On 3/25/26 09:21, Harry Yoo (Oracle) wrote:
> > On Wed, Mar 25, 2026 at 08:50:07AM +0100, Vlastimil Babka (SUSE) wrote:
> >> On 3/24/26 22:35, Jann Horn wrote:
> >> > Disable CONFIG_KVFREE_RCU_BATCHED in CONFIG_RCU_STRICT_GRACE_PERIOD builds
> >> > so that kernel fuzzers have an easier time finding use-after-free involving
> >> > kfree_rcu().
> >> >
> >> > The intent behind CONFIG_RCU_STRICT_GRACE_PERIOD is that RCU should invoke
> >> > callbacks and free objects as soon as possible (at a large performance
> >> > cost) so that kernel fuzzers and such have an easier time detecting
> >> > use-after-free bugs in objects with RCU lifetime.
> >> >
> >> > CONFIG_KVFREE_RCU_BATCHED is a performance optimization that queues
> >> > RCU-freed objects in ways that CONFIG_RCU_STRICT_GRACE_PERIOD can't
> >> > expedite; for example, the following testcase doesn't trigger a KASAN splat
> >> > when CONFIG_KVFREE_RCU_BATCHED is enabled:
> >> > ```
> >> > struct foo_struct {
> >> > struct rcu_head rcu;
> >> > int a;
> >> > };
> >> > struct foo_struct *foo = kmalloc(sizeof(*foo),
> >> > GFP_KERNEL | __GFP_NOFAIL | __GFP_ZERO);
> >> >
> >> > pr_info("%s: calling kfree_rcu()\n", __func__);
> >> > kfree_rcu(foo, rcu);
> >> > msleep(10);
> >> > pr_info("%s: start UAF access\n", __func__);
> >> > READ_ONCE(foo->a);
> >> > pr_info("%s: end UAF access\n", __func__);
> >> > ```
> >> >
> >> > Signed-off-by: Jann Horn <jannh@google.com>
> >>
> >> Hm but with 7.0 we have sheaves everywhere including kmalloc caches, and
> >> there's a percpu rcu_free sheaf collecting kfree_rcu'd objects.
> >
> > Right, but only when CONFIG_KVFREE_RCU_BATCHED=y
> >
> >> Only when
> >> it's full it's submitted to call_rcu() where the callback rcu_free_sheaf()
> >> runs slab_free_hook() including kasan hooks etc. If there's nothing filling
> >> the rcu_free sheaf, the objects can sit there possibly indefinitely.
> >
> > Right.
> >
> >> That means CONFIG_KVFREE_RCU_BATCHED now handles only the rare cases where
> >> kfree_rcu() to the rcu_free sheaf fails (and I still owe it to Ulad to do
> >> something about this).
> >
> > Right.
> >
> >> So to complete the intent of this patch, we should perhaps also skip the
> >> rcu_free sheaf with RCU_STRICT_GRACE_PERIOD? (or with !KVFREE_RCU_BATCHED
> >> perhaps as it's also a form of batching).
> >
> > Maybe I'm missing something, but...
> >
> > by making KVFREE_RCU_BATCHED depend on !RCU_STRICT_GRACE_PERIOD,
> > selecting RCU_STRICT_GRACE_PERIOD disables all uses of rcu_free sheaves?
> >
> > kvfree_call_rcu() implementation on !KVFREE_RCU_BATCHED does not call
> > kfree_rcu_sheaf().
>
> Ah yeah, I missed that there are two kvfree_call_rcu() implementations and
> kfree_rcu_sheaf() is only used in the batched one. Sorry for the noise.
It's confusing indeed. I was trapped by this yesterday, thinking...
"Oh, why doesn't kvfree_rcu_barrier() on !KVFREE_RCU_BATCHED flush
rcu sheaves? It's broken!"
and then realized that I was confused :)
> Will queue the patch
Thanks!
--
Cheers,
Harry / Hyeonggon
On Tue, Mar 24, 2026 at 10:35:12PM +0100, Jann Horn wrote:
> Disable CONFIG_KVFREE_RCU_BATCHED in CONFIG_RCU_STRICT_GRACE_PERIOD builds
> so that kernel fuzzers have an easier time finding use-after-free involving
> kfree_rcu().
>
> The intent behind CONFIG_RCU_STRICT_GRACE_PERIOD is that RCU should invoke
> callbacks and free objects as soon as possible (at a large performance
> cost) so that kernel fuzzers and such have an easier time detecting
> use-after-free bugs in objects with RCU lifetime.
>
> CONFIG_KVFREE_RCU_BATCHED is a performance optimization that queues
> RCU-freed objects in ways that CONFIG_RCU_STRICT_GRACE_PERIOD can't
> expedite; for example, the following testcase doesn't trigger a KASAN splat
> when CONFIG_KVFREE_RCU_BATCHED is enabled:
> ```
> struct foo_struct {
> struct rcu_head rcu;
> int a;
> };
> struct foo_struct *foo = kmalloc(sizeof(*foo),
> GFP_KERNEL | __GFP_NOFAIL | __GFP_ZERO);
>
> pr_info("%s: calling kfree_rcu()\n", __func__);
> kfree_rcu(foo, rcu);
> msleep(10);
> pr_info("%s: start UAF access\n", __func__);
> READ_ONCE(foo->a);
> pr_info("%s: end UAF access\n", __func__);
> ```
>
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
Acked-by: Harry Yoo (Oracle) <harry@kernel.org>
--
Cheers,
Harry / Hyeonggon
On 3/24/2026 5:35 PM, Jann Horn wrote:
> Disable CONFIG_KVFREE_RCU_BATCHED in CONFIG_RCU_STRICT_GRACE_PERIOD builds
> so that kernel fuzzers have an easier time finding use-after-free involving
> kfree_rcu().
>
> The intent behind CONFIG_RCU_STRICT_GRACE_PERIOD is that RCU should invoke
> callbacks and free objects as soon as possible (at a large performance
> cost) so that kernel fuzzers and such have an easier time detecting
> use-after-free bugs in objects with RCU lifetime.
>
> CONFIG_KVFREE_RCU_BATCHED is a performance optimization that queues
> RCU-freed objects in ways that CONFIG_RCU_STRICT_GRACE_PERIOD can't
> expedite; for example, the following testcase doesn't trigger a KASAN splat
> when CONFIG_KVFREE_RCU_BATCHED is enabled:
> ```
> struct foo_struct {
> struct rcu_head rcu;
> int a;
> };
> struct foo_struct *foo = kmalloc(sizeof(*foo),
> GFP_KERNEL | __GFP_NOFAIL | __GFP_ZERO);
>
> pr_info("%s: calling kfree_rcu()\n", __func__);
> kfree_rcu(foo, rcu);
> msleep(10);
> pr_info("%s: start UAF access\n", __func__);
> READ_ONCE(foo->a);
> pr_info("%s: end UAF access\n", __func__);
> ```
>
> Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
On Tue, 24 Mar 2026, Jann Horn wrote:
> Disable CONFIG_KVFREE_RCU_BATCHED in CONFIG_RCU_STRICT_GRACE_PERIOD builds
> so that kernel fuzzers have an easier time finding use-after-free involving
> kfree_rcu().
>
> The intent behind CONFIG_RCU_STRICT_GRACE_PERIOD is that RCU should invoke
> callbacks and free objects as soon as possible (at a large performance
> cost) so that kernel fuzzers and such have an easier time detecting
> use-after-free bugs in objects with RCU lifetime.
>
> CONFIG_KVFREE_RCU_BATCHED is a performance optimization that queues
> RCU-freed objects in ways that CONFIG_RCU_STRICT_GRACE_PERIOD can't
> expedite; for example, the following testcase doesn't trigger a KASAN splat
> when CONFIG_KVFREE_RCU_BATCHED is enabled:
> ```
> struct foo_struct {
> struct rcu_head rcu;
> int a;
> };
> struct foo_struct *foo = kmalloc(sizeof(*foo),
> GFP_KERNEL | __GFP_NOFAIL | __GFP_ZERO);
>
> pr_info("%s: calling kfree_rcu()\n", __func__);
> kfree_rcu(foo, rcu);
> msleep(10);
> pr_info("%s: start UAF access\n", __func__);
> READ_ONCE(foo->a);
> pr_info("%s: end UAF access\n", __func__);
> ```
>
> Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: David Rientjes <rientjes@google.com>
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