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Message-ID: <597b358e-3b45-4fba-922d-31208b55d15a@amd.com> Date: Wed, 14 Aug 2024 13:33:58 +0100 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 2/4] xen: make VMAP only support in MMU system Content-Language: en-GB To: Jan Beulich , Ayan Kumar Halder Cc: xen-devel@lists.xenproject.org, Penny Zheng , Wei Chen , sstabellini@kernel.org, bertrand.marquis@arm.com, michal.orzel@amd.com, Volodymyr_Babchuk@epam.com, julien@xen.org References: <20240813171356.46760-1-ayan.kumar.halder@amd.com> <20240813171356.46760-3-ayan.kumar.halder@amd.com> <6d333d94-80ad-485c-b024-a45a388b96a4@suse.com> From: Ayan Kumar Halder In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR3P281CA0041.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:4a::17) To PH8PR12MB7326.namprd12.prod.outlook.com (2603:10b6:510:216::7) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR12MB7326:EE_|CYXPR12MB9338:EE_ X-MS-Office365-Filtering-Correlation-Id: e1b8d334-7a7e-4780-05bd-08dcbc5d61d5 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014; 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Since this is highly dependent on virtual >>>> address translation, we choose to fold VMAP in MMU system. >>>> >>>> In this patch, we introduce a new Kconfig CONFIG_HAS_VMAP, and make it >>>> only support in MMU system on ARM architecture. And ALTERNATIVE now >>>> depends on VMAP. >>>> >>>> HARDEN_BRANCH_PREDICTOR is now gated on HAS_VMAP as speculative >>>> attacks are not possible on non MMU based systems (ie Cortex-R52, R82). >>>> See https://developer.arm.com/Arm%20Security%20Center/Speculative%20Processor%20Vulnerability. >>> While I'm not an Arm expert and hence I'm likely missing aspects, I question >>> the one (Spectre-BHB) vulnerability there to be sufficient to draw a >>> conclusion towards the usefulness of branch hardening. I would advise >>> against encoding such a connection in the Kconfig dependencies. >> AFAIU, to address 'Spectre' like vulnerabilities 'branch hardening' was >> added. >> >> See https://lore.kernel.org/all/E1fNadD-0000fz-9r@rmk-PC.armlinux.org.uk/ >> >> And from >> https://lists.linaro.org/archives/list/linux-stable-mirror@lists.linaro.org/message/F4MGL4WT2R7T54NLRDGKFRQNSXF3DZGD/ >> >> Spectre is valid on MMU based systems. > Since then various other issues / flavors were found. I've been focusing > on the x86 side of things, but I'd be very surprised if some didn't > affect other architectures as well. We are talking about Arm here as "HARDEN_BRANCH_PREDICTOR" is specific to Arm. https://developer.arm.com/Arm%20Security%20Center/Speculative%20Processor%20Vulnerability covers all the flavours and it does not include Cortex-R82 or R52. It says the following :- "Cortex-R cores typically use a closed software stack. In those environments, applications or processes are strictly controlled, and therefore not exploitable" > Plus branch hardening can be a pre- > cautionary measure, too, I think. The first two Arm non MMU cores that we wish to support in the forthcoming series is Cortex-R82 and R52. As seen in https://developer.arm.com/documentation/ka005109/latest/, it explicitly states the following about R82 The Cortex-R82 implements the faulting feature (FEAT_FPAC) but is not vulnerable. The Cortex-R82 behaves differently than vulnerable A-class CPUs when speculatively executing past an instruction that authenticates PAC, and that behavior does not allow the attack software to create the "oracle". We can re-enable branch hardening if we know there is a valid non MMU Arm core which is vulnerable. Let me know if you are ok with the rationale. - Ayan > > Jan