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Fri, 1 Aug 2025 04:43:12 +0000 Message-ID: <1c42680f-c962-4a75-998f-031008dd8343@intel.com> Date: Thu, 31 Jul 2025 21:43:10 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv9 04/16] x86/cpu: Defer CR pinning setup until core initcall Content-Language: en-US To: Dave Hansen , Thomas Gleixner , Dave Hansen , Kees Cook CC: Jonathan Corbet , Ingo Molnar , "Pawan Gupta" , Daniel Sneddon , Kai Huang , "Sandipan Das" , Breno Leitao , Rick Edgecombe , Alexei Starovoitov , Hou Tao , Juergen Gross , Vegard Nossum , Eric Biggers , "Jason Gunthorpe" , "Masami Hiramatsu (Google)" , Andrew Morton , Luis Chamberlain , Yuntao Wang , Rasmus Villemoes , Christophe Leroy , Tejun Heo , Changbin Du , Huang Shijie , Geert Uytterhoeven , Namhyung Kim , "Arnaldo Carvalho de Melo" , , , , , "Kirill A. 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McKenney" , Josh Poimboeuf , Xiongwei Song , Xin Li , "Mike Rapoport (IBM)" , Brijesh Singh , Michael Roth , Tony Luck , Alexey Kardashevskiy , Alexander Shishkin , X86-kernel References: <20250707080317.3791624-1-kirill.shutemov@linux.intel.com> <20250707080317.3791624-5-kirill.shutemov@linux.intel.com> <6075af69-299f-43d2-a3c8-353a2a3b7ee7@intel.com> <98a7a91b-3b46-4407-82a7-5f80443b7e00@intel.com> <6e768f25-3a1c-48b9-bc53-56877a556a83@intel.com> From: Sohil Mehta In-Reply-To: <6e768f25-3a1c-48b9-bc53-56877a556a83@intel.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BYAPR02CA0044.namprd02.prod.outlook.com (2603:10b6:a03:54::21) To BYAPR11MB3320.namprd11.prod.outlook.com (2603:10b6:a03:18::25) 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: BYAPR11MB3320:EE_|CH2PR11MB8868:EE_ X-MS-Office365-Filtering-Correlation-Id: 761ae324-91b8-451b-b667-08ddd0b5ec2e X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|7416014; 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Secondary CPUs should > also read 'cr4_pinned_bits' when setting up their own cr4's, > unconditionally, independent of 'cr_pinning'. > > The thing I think we should change is the pinning _enforcement_. The > easiest way to do that is to remove the static_branch_likely() in > cr4_init() and then delay flipping the static branch until just before > userspace starts. > Based on the current implementation and some git history [1], it seems that cr_pinning is expected to catch 2 types of issues. One is a malicious user trying to change the CR registers from userspace. But, another scenario is a regular caller to native_write_cr4() *mistakenly* clearing a pinned bit. [1]: https://lore.kernel.org/all/alpine.DEB.2.21.1906141646320.1722@nanos.tec.linutronix.de/ Could deferring enforcement lead to a scenario where we end up with different CR4 values on different CPUs? Maybe I am misinterpreting this and protecting against in-kernel errors is not a goal. In general, you want to delay the CR pinning enforcement until absolutely needed. I am curious about the motivation. I understand we should avoid doing it at arbitrary points in the boot. But, arch_cpu_finalize_init() and early_initcall() seem to be decent mileposts to me. Are you anticipating that we would need to move setup_cr_pinning() again when another user similar to EFI shows up? > Basically, split up the: > > static void __init setup_cr_pinning(void) > { > cr4_pinned_bits = this_cpu_read(cpu_tlbstate.cr4) & cr4_pinned_mask; > static_key_enable(&cr_pinning.key); > } > > code into its two logical pieces: > > 1. Populate 'cr4_pinned_bits' from the boot CPU so the secondaries can > use it > 2. Enable the static key so pinning enforcement is enabled