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Bae" Subject: Re: [PATCH v8 11/20] KVM: x86: Handle EGPR index and REX2-incompatible opcodes To: CC: References: <20260930210750.1487547-1-chang.seok.bae@intel.com> <20260930210750.1487547-12-chang.seok.bae@intel.com> <20260930214736.1FC1E1F00899@smtp.kernel.org> Content-Language: en-US In-Reply-To: <20260930214736.1FC1E1F00899@smtp.kernel.org> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BY1P220CA0003.NAMP220.PROD.OUTLOOK.COM (2603:10b6:a03:59d::10) To DS0PR11MB7925.namprd11.prod.outlook.com (2603:10b6:8:f8::18) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB7925:EE_|BL3PR11MB6411:EE_ X-MS-Office365-Filtering-Correlation-Id: cfe700e1-9f0e-4da2-bee9-08df1ffa44cc X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016|23010399003|10067099003|56012099006|11063799006|3023799007|4143699003|18002099003|22082099003; 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>> } >> >> + /* >> + * Instructions marked with NoRex2 ignore a legacy REX prefix, but #UD >> + * should be raised when prefixed with REX2. >> + */ >> + if (opcode.flags & NoRex2 && ctxt->rex_prefix == REX2_PREFIX) >> + opcode.flags = Undefined; >> + > > [Severity: High] > Does evaluating opcode.flags & NoRex2 here occur too early? > > Since this check precedes the while (ctxt->d & GroupMask) loop that > expands opcode groups, will nested group instructions (like monitor, > mwait, or vmcall) bypass this check and fail to raise #UD when given > a REX2 prefix? The byte following a REX2 prefix is interpreted as the opcode. The emulator then handles it as a legacy instruction, including raising #UD where required, rather than suppressing #UD merely because of being REX2-prefixed. The flag intentionally marks the reserved opcode ranges defined by the specification: * Legacy map 0: rows 0x4*, 0x7*, 0xA*, 0xE* * Legacy map 1: rows 0x3*, 0x8* This was also pointed out in the previouss discussion: https://lore.kernel.org/kvm/6bdc1a64-0304-4845-a36c-5f309b0b40e0@intel.com/ Thanks, Chang