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Thu, 22 Jan 2026 05:44:34 +0000 From: Date: Wed, 21 Jan 2026 21:44:32 -0800 To: Jason Gunthorpe , Jonathan Cameron CC: "Tian, Kevin" , Nicolin Chen , "will@kernel.org" , "robin.murphy@arm.com" , "bhelgaas@google.com" , "Williams, Dan J" , "joro@8bytes.org" , "praan@google.com" , "baolu.lu@linux.intel.com" , "miko.lenczewski@arm.com" , "linux-arm-kernel@lists.infradead.org" , "iommu@lists.linux.dev" , "linux-kernel@vger.kernel.org" , "linux-pci@vger.kernel.org" , Message-ID: <6971b9406d069_1d33100df@dwillia2-mobl4.notmuch> In-Reply-To: <20260121130315.GE1134360@nvidia.com> References: <20260121100307.00004e60@huawei.com> <20260121130315.GE1134360@nvidia.com> Subject: Re: [PATCH RFCv1 1/3] PCI: Allow ATS to be always on for CXL.cache capable devices Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BYAPR01CA0046.prod.exchangelabs.com (2603:10b6:a03:94::23) To PH8PR11MB8107.namprd11.prod.outlook.com (2603:10b6:510:256::6) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR11MB8107:EE_|PH3PPF801A91A7D:EE_ X-MS-Office365-Filtering-Correlation-Id: a0ae99d7-a475-4e36-7bcd-08de59795243 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|7416014|376014|1800799024; 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I recalled an offline discussion in which he raised concern on > > > having the kernel blindly enable ATS for cxl.cache device instead of > > > creating a knob for admin to configure from userspace (in case > > > security is viewed more important than functionality, upon allowing > > > DMA to read data out of CPU caches)... > > > > > > > +CC Linux-cxl > > A cxl.cache device supporting ATS will automatically enable ATS today > if the kernel option to enable translation is set. > > Even if the device is marked untrusted by the PCI layer (eg an > external port). > > Yes this is effectively a security issue, but it is not really a CXL > specific problem. My contention is that it is a worse or at least different problem in the CXL case because now you have a new toolkit in an attack that wants to exfiltrate data from CPU caches. > We might perfer to not enable ATS for untrusted devices and then fail to > load drivers for "ats always on" cases. The current PCI untrusted flag is not fit for purpose in this new age of PCI device authentication and CXL.cache capable devices. > Or maybe we can enable one of the ATS security features someday, > though I wonder if those work for CXL.. It should work, but before that I do not see the justification to say effectively: "We have a less than perfect legacy way (PCI untrusted flag) to nod at ATS security problems. Let us ignore even that for a new class of devices that advertise they can trigger all the old security problems plus new ones." I do not immediately see what is wrong with requiring userspace policy opt-in. That naturally gets replaced by installing the device's certificate (for native PCI CMA), authenticating the device with the TSM (for PCI IDE), or obviated by secure-ATS if that arrives.