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Mon, 22 Sep 2025 22:43:10 +0000 From: Date: Mon, 22 Sep 2025 15:43:08 -0700 To: Laurent Pinchart , Danilo Krummrich CC: Bartosz Golaszewski , Greg Kroah-Hartman , Tzung-Bi Shih , "Bartosz Golaszewski" , Krzysztof Kozlowski , Benson Leung , "Rafael J . Wysocki" , Jonathan Corbet , Shuah Khan , Dawid Niedzwiecki , , , , , "Wolfram Sang" , Dan Williams Message-ID: <68d1d0fc5ef09_1c79100e@dwillia2-mobl4.notmuch> In-Reply-To: <20250913161717.GE4842@pendragon.ideasonboard.com> References: <20250912132656.GC31682@pendragon.ideasonboard.com> <2025091209-curfew-safari-f6e0@gregkh> <20250912135916.GF31682@pendragon.ideasonboard.com> <2025091220-private-verse-d979@gregkh> <20250912142646.GI31682@pendragon.ideasonboard.com> <2025091237-cortex-carnage-5c34@gregkh> <20250912145416.GL31682@pendragon.ideasonboard.com> <1dc06657-e136-45c2-8012-9199194bfc9b@kernel.org> <20250913161717.GE4842@pendragon.ideasonboard.com> Subject: Re: [PATCH v3 0/5] platform/chrome: Fix a possible UAF via revocable Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: BY3PR04CA0022.namprd04.prod.outlook.com (2603:10b6:a03:217::27) To PH8PR11MB8107.namprd11.prod.outlook.com (2603:10b6:510:256::6) Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR11MB8107:EE_|BL3PR11MB6483:EE_ X-MS-Office365-Filtering-Correlation-Id: 16e6f985-03d2-47df-f23e-08ddfa2967f9 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|7416014|1800799024; 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We have other structures that want to do this type of thi= ng > > >>> today as is proof with the rust implementation for the devm api. > > >> > > >> Yeah, I'm not against this going upstream. If more development is > > >> needed for this to be usable in other parts of the kernel, that can = be > > >> done gradually. Literally no subsystem ever was perfect on day 1. > > >=20 > > > To be clear, I'm not against the API being merged for the use cases t= hat > > > would benefit from it, but I don't want to see drivers using it to > > > protect from the cdev/unregistration race. > >=20 > > I mean, revocable is really a synchronization primitive in the end that > > "revokes" access to some resource in a race free way. > >=20 > > So, technically, it probably belongs into lib/. > >=20 > > I think the reason it ended up in drivers/base/ is that one common use = case is > > to revoke a device resource from a driver when the device is unbound fr= om this > > driver; or in other words devres is an obvious user. > >=20 > > So, I think that any other API (cdev, devres, etc.) should be built on= top of it. >=20 > No issue with that. I'm sure there are people who have better knowledge > than me when it comes to implementing the low-level primitive in the > most efficient way. What I have lots of experience with is the impact of > API design on drivers, and the API misuse (including through cargo-cult > programming) this can generate. Let's design the API towards drivers > correctly. Note that I dropped the "managed_fops" [1] effort, targeted for use for CXL, in favor of simply this in the CXL ioctl device shutdown path: cdev_device_del(&cxlmd->cdev, &cxlmd->dev); scoped_guard(rwsem_write, &cxl_memdev_rwsem) cxlmd->cxlds =3D NULL; put_device(&cxlmd->dev); Pair that with: guard(rwsem_read)(&cxl_memdev_rwsem); cxlds =3D cxlmd->cxlds; if (cxlds) return __cxl_memdev_ioctl(cxlmd, cmd, arg); return -ENXIO; ...on the ioctl invocation side. This "revocable" mechanism looks useful for other inter-driver resource sharing, but not for the well known issues with cdev. For cdev and the design pattern of "shutdown the ioctl path on a core-subsystem device object that is also a chardev", just use cdev_device_add() with an rwsem. [1]: http://lore.kernel.org/all/CAPcyv4h74NjqcuUjv4zFKHAxio_bV0bngLoxP=3DAC= w=3DJvMfq-UA@mail.gmail.com=