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Tue, 4 May 2021 12:31:24 +0000 Cc: brijesh.singh@amd.com, x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, tglx@linutronix.de, bp@alien8.de, jroedel@suse.de, Thomas.Lendacky@amd.com, pbonzini@redhat.com, mingo@redhat.com, rientjes@google.com, seanjc@google.com, peterz@infradead.org, hpa@zytor.com, tony.luck@intel.com Subject: Re: [PATCH Part2 RFC v2 10/37] x86/fault: Add support to handle the RMP fault for kernel address To: Dave Hansen , Andy Lutomirski References: <9e3e4331-2933-7ae6-31d9-5fb73fce4353@amd.com> <40C2457E-C2A3-4DF7-BD16-829D927CC17C@amacapital.net> <1c98a55a-d4d5-866e-dcad-81caa09a495d@amd.com> From: Brijesh Singh Message-ID: Date: Tue, 4 May 2021 07:31:20 -0500 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.10.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Originating-IP: [165.204.77.11] X-ClientProxiedBy: SN6PR16CA0061.namprd16.prod.outlook.com (2603:10b6:805:ca::38) To SN6PR12MB2718.namprd12.prod.outlook.com (2603:10b6:805:6f::22) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from Brijeshs-MacBook-Pro.local (165.204.77.11) by SN6PR16CA0061.namprd16.prod.outlook.com (2603:10b6:805:ca::38) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4108.24 via Frontend Transport; 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Should I drop >> this patch or keep it just in the case we miss something? > I think you should drop it, and just ensure that the existing page fault > oops code can produce a coherent, descriptive error message about what > went wrong. A malicious guest could still trick the host into accessing a guest private page unless we make sure that host kernel *never* does kmap() on GPA. The example I was thinking is: 1. Guest provides a GPA to host. 2. Host queries the RMP table and finds that GPA is shared and allows the kmap() to happen. 3. Guest later changes the page to private. 4. Host write to mapped address will trigger a page-fault. KVM provides kvm_map_gfn(), kvm_vcpu_map() to map a GPA; these APIs will no longer be safe to be used. In addition, some shared pages are registered once by the guest and KVM updates the contents of the page on vcpu enter (e.g, CPU steal time). IMHO, we should add the RMP table check before kmap'ing GPA but still keep this patch to mitigate the cases where a malicious guest changes the page state after the kmap(). -Brijesh