[PATCH] slab,rcu: disable KVFREE_RCU_BATCHED for strict grace period

[Jann Horn <jannh@google.com>]

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>