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Message-ID: <0e846638-c23c-0ec7-54b8-e318527ffac5@suse.com> Date: Wed, 24 Aug 2022 08:00:50 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [PATCH] x86/CPUID: surface suitable value in EBX of XSTATE subleaf 1 Content-Language: en-US To: Andrew Cooper Cc: Wei Liu , Roger Pau Monne , =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= , Thomas Gleixner , "xen-devel@lists.xenproject.org" References: <27f97ef0-148d-a0c3-ae13-0ed4d640a10c@citrix.com> From: Jan Beulich In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: AM6PR05CA0034.eurprd05.prod.outlook.com (2603:10a6:20b:2e::47) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 9b664c90-3386-4170-8e63-08da859600af X-MS-TrafficTypeDiagnostic: VI1PR04MB4413:EE_ X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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The necessary fiddling with XSS >> here would of course be dependent upon p->xstate.xsaves alone (or, >> maybe better, on the set of enabled features in XSS being non-empty), >> but that's simply another (inner) if(). >> >> As an aside, I actually wonder what use the supplied size is to user >> mode code when any XSS-controlled feature is enabled: They'd allocate >> a needlessly large block of memory, as they would only be able to use >> XSAVEC. > > This field is an already known kernel=>user infoleak.  There are threads > about it on LKML. > > But it does highlight another problem.  This change does not fix Linux > on AMD Zen3 hardware, where the kernel will find the CPUID value larger > than it can calculate the size to be, because Xen's use of CET-SS will > show up in the CPUID value. Why would that be? We don't even have CET related defines for XCR0, so we don't enable the states in XSS. And I don't see why we would. Even for other XSTATE-managed but not XSTATE-enabled features we could clear the respective bits around the CPUID invocation (just like we may need to set some in XSS). We'd be in trouble only for any XSTATE- enabled feature that we make use of ourselves. Jan