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Tue, 24 Mar 2026 14:35:20 -0700 (PDT) Received: from localhost ([2a00:79e0:288a:8:ded1:e23b:8c0a:3b21]) by smtp.gmail.com with UTF8SMTPSA id ffacd0b85a97d-43b8aa474b6sm710974f8f.10.2026.03.24.14.35.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 24 Mar 2026 14:35:19 -0700 (PDT) From: Jann Horn Date: Tue, 24 Mar 2026 22:35:12 +0100 Subject: [PATCH] slab,rcu: disable KVFREE_RCU_BATCHED for strict grace period Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260324-kasan-kfree-rcu-v1-1-ac58a7a13d03@google.com> X-B4-Tracking: v=1; b=H4sIAI8Dw2kC/x3MwQpAQBRG4VfRXbs1xkS8iiwm/uGmhu5ESt7dZ PktznkoQQWJ+uIhxSVJ9phRlQVNq48LWOZsssY2praON5985C0owDqd7NAG1xlbtwiUq0MR5P6 Pw/i+H49oAKphAAAA X-Change-ID: 20260324-kasan-kfree-rcu-4e7f490237ef To: Vlastimil Babka , Harry Yoo , Andrew Morton Cc: Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , "Paul E. McKenney" , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Dmitry Vyukov , rcu@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jann Horn X-Mailer: b4 0.15-dev X-Developer-Signature: v=1; a=ed25519-sha256; t=1774388116; l=1651; i=jannh@google.com; s=20240730; h=from:subject:message-id; bh=usnnnH7wUGYWY5NIsmoimFYx19G6HqPPrRI2oZAKt1Q=; b=SgmquGvg0X9VR0OP0g3btEDHfyjT6PMsoHAYVyfCmCV0ATxqjDCX7hohdeoRiLQJIAjY7BLQY dwd1J12BZ0kD7ik8LI0ZUha0325jzs7B3OE7eJaqSFXoG05/MNUX12w X-Developer-Key: i=jannh@google.com; a=ed25519; pk=AljNtGOzXeF6khBXDJVVvwSEkVDGnnZZYqfWhP1V+C8= 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 --- 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