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Thu, 16 Oct 2025 18:17:54 +0000 Message-ID: <47b9871c-6c08-4bac-bf05-2e6f431beb95@intel.com> Date: Thu, 16 Oct 2025 11:17:51 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] x86,fs/resctrl: Fix NULL pointer dereference when events force disabled while in mbm_event mode To: Babu Moger , , , , , , , CC: , , , , , References: From: Reinette Chatre Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR03CA0107.namprd03.prod.outlook.com (2603:10b6:303:b7::22) To SJ2PR11MB7573.namprd11.prod.outlook.com (2603:10b6:a03:4d2::10) 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: SJ2PR11MB7573:EE_|SA1PR11MB8810:EE_ X-MS-Office365-Filtering-Correlation-Id: 9e06eb52-0046-497d-1c58-08de0ce052f9 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|366016|1800799024; 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This results in the > per-domain MBM event related data structures to not be allocated during > resctrl early initialization. > > resctrl fs initialization follows by implicitly enabling both MBM total and > local events on a system that supports assignable counters (mbm_event > mode), but this enabling occurs after the per-domain data structures have > been created. > > During runtime resctrl fs assumes that an enabled event can access all its > state. This results in NULL pointer dereference when resctrl attempts to > access the un-allocated structures of an enabled event. > > Remove the late MBM event enabling from resctrl fs. > > This leaves a problem where the X86_FEATURE_CQM_MBM_TOTAL and > X86_FEATURE_CQM_MBM_LOCAL features may be disabled while assignable > counter (mbm_event) mode is enabled without any events to support. > Switching between the "default" and "mbm_event" mode without any events > is not practical. > > Create a dependency between the > X86_FEATURE_CQM_MBM_TOTAL/X86_FEATURE_CQM_MBM_LOCAL and X86_FEATURE_ABMC > (assignable counter) hardware features. An x86 system that supports > assignable counters now requires support of X86_FEATURE_CQM_MBM_TOTAL or > X86_FEATURE_CQM_MBM_LOCAL. This ensures all needed MBM related data > structures are created before use and that it is only possible to switch > between "default" and "mbm_event" mode when the same events are available > in both modes. This dependency does not exist in the hardware but this > usage of these feature settings work for known systems. > > Fixes: 13390861b426e ("x86,fs/resctrl: Detect Assignable Bandwidth Monitoring feature details") > Co-developed-by: Reinette Chatre > Signed-off-by: Reinette Chatre > Signed-off-by: Babu Moger > --- Thank you very much. Reviewed-by: Reinette Chatre Reinette