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Fri, 11 Apr 2025 21:48:27 +0000 Date: Fri, 11 Apr 2025 14:48:23 -0700 From: Dan Williams To: Dave Hansen , Dan Williams , Kees Cook CC: , Nikolay Borisov , Subject: Re: [PATCH v2 2/3] devmem: Block mmap access when read/write access is restricted Message-ID: <67f98e27a799e_7205294e3@dwillia2-xfh.jf.intel.com.notmuch> References: <174433453526.924142.15494575917593543330.stgit@dwillia2-xfh.jf.intel.com> <174433455090.924142.10859114556652046264.stgit@dwillia2-xfh.jf.intel.com> <202504101926.0F8FB73@keescook> <67f8a1a15cc29_7205294d7@dwillia2-xfh.jf.intel.com.notmuch> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: MW4PR03CA0252.namprd03.prod.outlook.com (2603:10b6:303:b4::17) To PH8PR11MB8107.namprd11.prod.outlook.com (2603:10b6:510:256::6) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR11MB8107:EE_|MN6PR11MB8241:EE_ X-MS-Office365-Filtering-Correlation-Id: afae3d50-ee3d-40dc-c646-08dd79429742 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|1800799024; 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So I feel ok making TVMs take the modern > > replacement path which is what they would need to do anyway in the > > lockdown_kernel case. Tom, Dave, what do you think? > > Yeah, doing the same as lockdown should be fine. Note that lockdown fails the open() for /dev/mem and fails the mmap() for PCI sysfs. So the proposal here would arrange for lockdown to not be required, but fail mmap() in both cases. > The other alternative to failing all mmap()s is to allow mmap(PROT_READ) > to succeed and then just map the zero page. The only goal of mapping zeroes I can see is to attempt to break legacy userspace less severely, but as the Debian code search shows, legacy /dev/mem users have already found the mmap() loophole. So, zeroes for the TVM case does not help satisfy the requirement to use /sys/firmware/dmi/tables and other modern methods with private-memory-safe semantics. I.e. the "success but zero" and "mmap() failed" cases have the same outcome: legacy software falls back to no DMI info. For PCI sysfs resource mmap() the semantics are different. mmap() fails only when devmem_is_allowed() says "no" *and* the kernel has marked the range as IORESOURCE_BUSY in the iomem resource tree. That should be sufficient to allow userpsace PCI drivers in TVMs because the goal here is to avoid simultaneous mappings with mismatched encryption settings or allowing userspace access to unaccepted private memory. As long as the upcoming TDISP code is careful to hold a request_resource() reservation over attempts to convert PCI MMIO from shared to private, it should close any potential for mismatched encryption settings, or access to unaccepted private MMIO. However to get that behavior of not allowing simultaneous kernel-ioremap() plus userspace mmap() of a PCI resource the kernel needs to be built with CONFIG_IO_STRICT_DEVMEM=y. I note, for example that RHEL does not set that, but Fedora does.