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Dadhania" , Borislav Petkov , Dave Hansen , Paolo Bonzini , Kishon Vijay Abraham I References: <20250207233410.130813-1-kim.phillips@amd.com> <20250207233410.130813-3-kim.phillips@amd.com> <4eb24414-4483-3291-894a-f5a58465a80d@amd.com> <6829cf75-5bf3-4a89-afbe-cfd489b2b24b@amd.com> Content-Language: en-US From: Kim Phillips Organization: AMD In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SN4PR0501CA0113.namprd05.prod.outlook.com (2603:10b6:803:42::30) To DS7PR12MB6263.namprd12.prod.outlook.com (2603:10b6:8:95::17) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB6263:EE_|MW4PR12MB7239:EE_ X-MS-Office365-Filtering-Correlation-Id: 60a6e9f5-5239-45bd-2107-08dd4d42d937 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014; X-Microsoft-Antispam-Message-Info: =?utf-8?B?ZTNhU1IxOU4ydGdKcEpYZUZTU0pYeThiRHh5Zlc0L3R2SXBqa2swcHcyVlFP?= =?utf-8?B?VHJCaTNlaDA4U2Zlemp6UVNYanVHWmVZRzF5YkdVWE93M3NrWGJRMmthSkxv?= =?utf-8?B?QjZnR0V1Zi9IQnVBTkxyWnFlU3pBU1hQZ1l3MCtKbjJDclJuQnhnTnY5dGZZ?= =?utf-8?B?SjFNSm9vMGFkR29waUd6dzB0VERTM1ZKRm1yTjV6NFRaalRSRktIckx6SEQw?= =?utf-8?B?LzZNanZ2MWdEY1FKcVJSSFpHckJWN0wvYkJjeW5SMS9WSTYvRzdNN0ljclR5?= =?utf-8?B?bFBNNGJDR1Z1aWZ1c3l0NFBxVDJZUDhVOXVlTTZVbzJmQUc2eGNoQzRiRWV2?= =?utf-8?B?ZVFJRzd4VHlaT0hkOE9wZkMvNER5bHRyd3pXT1RiMTJsSWliSlB3VEltRmk1?= =?utf-8?B?Tk5ESmJxbnNJeGZmcEE2U3BrTHVQTHFUbW9DeE9LWE1xUWZVVFpiMWJzdTBH?= =?utf-8?B?cVBpK05wNWV4cndIb0wwMTNMQ1liekxocWNPZUNvVG1FVThpOFp1T3hPTzNh?= =?utf-8?B?RjZobWx0RFRYYlg1Z2ZjWWwwcEFsL0lNZDQvUmZnKzRkVFFNTkdOWTdiK3Ba?= =?utf-8?B?YjRpZjEydll6aG9lRE45UVhlWnlnUVVibjJHUktiOHBwdE9Oc1lud09RY0p4?= =?utf-8?B?Z0tQNE9EVEZDWHBTR0dqWkMwVnhvU2RvQ0JIVVdFNHowRzJ0cU5BUUNHVHg5?= =?utf-8?B?eFE5VEVCcDNoVElMaG9KNThlbEE0TWorL1VQeU9XWUVnTUh5WmxvMURKUm1B?= =?utf-8?B?VEJXNTRqa1N0Yk9lc3kyQ2hpSnBON0tpaGxNbUJHSUk3QkM4RGtWR2dYdHMw?= =?utf-8?B?WUdsMkg1d3JBcFhjblMwOHpySks1T2tSSjZDc0pwRjFzS2FqYTU0Y0RMTEhs?= =?utf-8?B?Mk1zaEE3ZVZvVytrbDEvZEhoV0dKY21DZDVyVU8zQlhrMExvK3Y4NU03TWpI?= =?utf-8?B?NDVxTWJkdUdOOHpXVi80WFgwZ1BmOGk2czJnbERLQ05nSHVhNVlMOC9SRmJX?= =?utf-8?B?MlljNDI3MWgrZDRUSWtjSzh4OTFzcTJtN05jRVVUQjk4eHJacmZRSDBLVmtP?= =?utf-8?B?QjVvSjVETmRtTXZ2YlVkY0t6cUdXVkdpRi82WmZETUprUHQ2U3RxN3FoL0JX?= =?utf-8?B?VlJ3WmJldCtVTXVYSjh6MGwrY0o2OGtwQkJNUmVTV3dZamFJeFlXTHRZRjZ0?= =?utf-8?B?TE84dlpJK05ZYnIzd1hJQlZybytMdnlIMHNZNjhpYXFXVEFsR3RST0NHTG9s?= =?utf-8?B?T1RaL1ByQUt5NmNUTnJiUXo4T3pWNmhlcjllSVR2Sk8rWmU2WjI2dEhMcnRn?= =?utf-8?B?c2pKUnVEVkhWWW4vTHdlTVlKYUNIY0lWaEpvUUxJZ0ZOQWxUVjZpWU52eWNI?= =?utf-8?B?MU5Ldk1CaEw5cmZ1U25FWWNQc3dreFVFMWZ0azZYUFpCWFRWRGg5UjR3N3BI?= =?utf-8?B?RDFrNGptaDdlTEpkelhEOXRtWE9jbzlOY2dkejNLaEFMaHp6TW9HRmtNb2dG?= =?utf-8?B?dUJPYVRCV0NacDN2K0JFTHhIN3FTRFVSYjJPcFVodmFCMGdVTFBBZndobGdx?= =?utf-8?B?L1pQdlN0WWZ2bHRBd0l3UlN2cGc1R0pNb2U1UE1ubktFQ1NMLzhYUEhUa1VX?= =?utf-8?B?d1A3SHUyM0lmVjd6cnRkdHc3Y0pNRE0xMy8wUFlmd2c3ajkzcXpoQ0RIZnJx?= =?utf-8?B?WUd1eWxiTjNyN1p1Yk90b3VqbG1Lb2hzSUdmalNKQkZFR1JxUlYxUzl5Tktr?= =?utf-8?B?QzVmdHV2UHFNQjBjVjNpendMNXgwdjZiOUhzU1lFYmFuMldvRUVkYmpybnFW?= =?utf-8?B?RVhEc2tFM1p6RDlwa3pMVTdHS3ZWT2FxZ3hXeTVOb1FWMXZHa0xDeU04a3Zr?= =?utf-8?B?ZzBOcGhlSnUybGxGRVFYNklRcEV3cnBKM3RVbUV3R2Jldnc9PQ==?= X-Forefront-Antispam-Report: CIP:255.255.255.255; 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envelope-from=kim.phillips@amd.com; helo=NAM04-BN8-obe.outbound.protection.outlook.com X-Spam_score_int: -37 X-Spam_score: -3.8 X-Spam_bar: --- X-Spam_report: (-3.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.732, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 2/13/25 6:55 PM, Sean Christopherson wrote: > On Thu, Feb 13, 2025, Kim Phillips wrote: >> On 2/11/25 3:46 PM, Sean Christopherson wrote: >>> On Mon, Feb 10, 2025, Tom Lendacky wrote: >>>> On 2/7/25 17:34, Kim Phillips wrote: >>>>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c >>>>> index a2a794c32050..a9e16792cac0 100644 >>>>> --- a/arch/x86/kvm/svm/sev.c >>>>> +++ b/arch/x86/kvm/svm/sev.c >>>>> @@ -894,9 +894,19 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm) >>>>> return 0; >>>>> } >>>>> +static u64 allowed_sev_features(struct kvm_sev_info *sev) >>>>> +{ >>>>> + if (cpu_feature_enabled(X86_FEATURE_ALLOWED_SEV_FEATURES) && >>>> >>>> Not sure if the cpu_feature_enabled() check is necessary, as init should >>>> have failed if SVM_SEV_FEAT_ALLOWED_SEV_FEATURES wasn't set in >>>> sev_supported_vmsa_features. >>> >>> Two things missing from this series: >>> >>> 1: KVM enforcement. No way is KVM going to rely on userspace to opt-in to >>> preventing the guest from enabling features. >>> 2: Backwards compatilibity if KVM unconditionally enforces ALLOWED_SEV_FEATURES. >>> Although maybe there's nothing to do here? I vaguely recall all of the gated >>> features being unsupported, or something... >> >> This contradicts your review comment from the previous version of the series [1]. > > First off, my comment was anything but decisive. I don't see how anyone can read > this and come away thinking "this is exactly what Sean wants". > > This may need additional uAPI so that userspace can opt-in. Dunno. I hope guests > aren't abusing features, but IIUC, flipping this on has the potential to break > existing VMs, correct? > > Second, there's a clear question there that went unanswered. Respond to questions > and elaborate as needed until we're all on the same page. Don't just send patches. > > Third, letting userspace opt-in to something doesn't necessarily mean giving > userspace full control. Which is the entire reason I asked the question about > whether or not this can break userspace. E.g. we can likely get away with only > making select features opt-in, and enforcing everything else by default. > > I don't think RESTRICTED_INJECTION or ALTERNATE_INJECTION can work without KVM > cooperation, so enforcing those shouldn't break anything. > > It's still not clear to me that we don't have a bug with DEBUG_SWAP. AIUI, > DEBUG_SWAP is allowed by default. I.e. if ALLOWED_FEATURES is unsupported, then > the guest can use DEBUG_SWAP via SVM_VMGEXIT_AP_CREATE without KVM's knowledge. > > So _maybe_ we have to let userspace opt-in to enforcing DEBUG_SWAP, but I suspect > we can get away with fully enabling ALLOWED_FEATURES without userspace's blessing. If I hardcode DEBUG_SWAP (bit 5) in the vmsa->sev_features assignment in wakeup_cpu_via_vmgexit(), such guest boots successfully with the kvm_amd module's debug_swap parameter set. The guest *doesn't* boot if I also turn on allowed_sev_features=1 with qemu and this patchseries. So, the answer is yes, always enforcing ALLOWED_SEV_FEATURES does break existing guests, thus the userspace opt-in for it. Thanks, Kim > >> If KVM enforces ALLOWED_SEV_FEATURES, it can break existing VMs, thus >> the explicit userspace allowed-sev-features=on opt-in [2]. >> >> Thanks, >> >> Kim >> >> [1] https://lore.kernel.org/kvm/ZsfKYHFkWA-Rh23C@google.com/ >> [2] https://lore.kernel.org/kvm/20250207233327.130770-1-kim.phillips@amd.com/