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McKenney" , 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 References: <20260324-kasan-kfree-rcu-v1-1-ac58a7a13d03@google.com> Content-Language: en-US From: Joel Fernandes In-Reply-To: <20260324-kasan-kfree-rcu-v1-1-ac58a7a13d03@google.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: CYZPR12CA0009.namprd12.prod.outlook.com (2603:10b6:930:8b::24) To DS0PR12MB6486.namprd12.prod.outlook.com (2603:10b6:8:c5::21) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR12MB6486:EE_|CYYPR12MB8889:EE_ X-MS-Office365-Filtering-Correlation-Id: c9a5c7fc-397b-41ae-3303-08de8a1b01b7 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|1800799024|7416014|22082099003|18002099003|7053199007|56012099003; 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 Reviewed-by: Joel Fernandes