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b=OsR5KBnjPJkle3PRLcgshxkxpg62pc0iUpMUoTdesdCTqB3NxGAauH3YmF/f9uQ0ROpheCcKYA39rWK4UROIaPkBBVOSjhYxeGY4+ni1MQPlzcTid/uNGhnbANLMn3vBLsECbezLghP55j4f61ASFNzUAxAi+YIZUVQHuWjEj9s= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from BL3PR12MB9049.namprd12.prod.outlook.com (2603:10b6:208:3b8::21) by SN7PR12MB7856.namprd12.prod.outlook.com (2603:10b6:806:340::16) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.406.8; Thu, 10 Sep 2026 22:46:08 +0000 Received: from BL3PR12MB9049.namprd12.prod.outlook.com ([fe80::ae6a:9bdd:af5b:e9ad]) by BL3PR12MB9049.namprd12.prod.outlook.com ([fe80::ae6a:9bdd:af5b:e9ad%5]) with mapi id 15.21.0406.007; Thu, 10 Sep 2026 22:46:08 +0000 Message-ID: <4d095cd6-a716-46aa-aff2-e7533d1511b0@amd.com> Date: Thu, 10 Sep 2026 17:46:07 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v14 2/5] x86/sev: Disable CPU hotplug while SNP is active To: sashiko-reviews@lists.linux.dev Cc: kvm@vger.kernel.org References: <20260910222518.AB5191F000FF@smtp.kernel.org> Content-Language: en-US From: "Kalra, Ashish" Autocrypt: addr=ashish.kalra@amd.com; 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Sashiko AI review found 1 potential issue(s) to consider: > - [High] System suspend/resume bypasses cpu_hotplug_disable() and brings CPUs online without SnpEn set, breaking the SNP invariant. > -- > > commit 0243c46e8f299e3de42fbcddbc4c7bd93fb68c65 > Author: Ashish Kalra > > x86/sev: Disable CPU hotplug while SNP is active > > This commit disables CPU hotplug while SEV-SNP is active. This ensures no > CPU can be brought online without the SnpEn bit set in MSR_AMD64_SYSCFG, > which is required for hardware RMP checks to function correctly. > >> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c >> index cff285d8ad8e3..558f7924a3f85 100644 >> --- a/arch/x86/virt/svm/sev.c >> +++ b/arch/x86/virt/svm/sev.c >> @@ -526,14 +525,18 @@ int snp_prepare(void) >> >> clear_rmp(); >> >> - cpus_read_lock(); >> + /* >> + * No CPU may come online without SnpEn while SNP is active; disable >> + * hotplug here and re-enable it in snp_shutdown(). >> + */ >> + cpu_hotplug_disable(); > > [Severity: High] > Does system suspend/resume bypass cpu_hotplug_disable() and bring CPUs online > without SnpEn set, breaking the SNP invariant? > > If the host initiates an ACPI system suspend (e.g., S3), freeze_secondary_cpus() > offlines all secondary CPUs via _cpu_down(), which bypasses the > cpu_hotplug_disabled counter incremented by cpu_hotplug_disable(). > > Upon resume, thaw_secondary_cpus() brings the CPUs back online via _cpu_up(). > Because CPU MSR state is lost during power-off in suspend and snp_enable() is > not registered as a CPU hotplug callback, the secondary CPUs wake up with SnpEn > cleared. This would allow them to bypass hardware RMP checks. > You're right that the suspend path bypasses cpu_hotplug_disable() — freeze_secondary_cpus() uses _cpu_down() directly and ignores the cpu_hotplug_disabled counter. But this is neither introduced nor affected by this patch, and does not apply to SNP hosts: 1. This patch does not touch the suspend/resume path. cpu_hotplug_disable() holds the online CPU set stable against the regular (sysfs) hotplug path while SNP is active. The cpus_read_lock() this patch replaces did not gate the freeze/thaw path either, so suspend/resume behaviour is unchanged from base SNP. 2. SNP hosts do not support system suspend/resume. It is not a supported configuration for an SNP‑enabled host, and there is no path today to re‑establish the per‑CPU SNP state (SnpEn) on resume — snp_enable() has never been registered as a CPU‑hotplug/resume callback. That is a pre‑existing property of base SNP, independent of this series. Thanks, Ashish