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Peter Anvin" , Ard Biesheuvel , Paolo Bonzini , Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Andy Lutomirski , Dave Hansen , Sergio Lopez , Peter Zijlstra , Srinivas Pandruvada , David Rientjes , Dov Murik , Tobin Feldman-Fitzthum , Borislav Petkov , Michael Roth , Vlastimil Babka , "Kirill A . Shutemov" , Andi Kleen , tony.luck@intel.com, Marc Orr , sathyanarayanan.kuppuswamy@linux.intel.com Subject: Re: [PATCH Part1 v5 34/38] x86/sev: Add snp_msg_seqno() helper To: Peter Gonda References: <20210820151933.22401-1-brijesh.singh@amd.com> <20210820151933.22401-35-brijesh.singh@amd.com> <4742dbfe-4e02-a7e3-6464-905ccc602e6c@amd.com> From: Brijesh Singh Message-ID: <7ddc78d4-745d-217e-11f1-77643863e6ee@amd.com> Date: Thu, 9 Sep 2021 14:26:49 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SA9P223CA0005.NAMP223.PROD.OUTLOOK.COM (2603:10b6:806:26::10) To SN6PR12MB2718.namprd12.prod.outlook.com (2603:10b6:805:6f::22) MIME-Version: 1.0 Received: from [10.236.31.95] (165.204.77.1) by SA9P223CA0005.NAMP223.PROD.OUTLOOK.COM (2603:10b6:806:26::10) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4500.16 via Frontend Transport; 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If thats >>> correct, that addresses my first concern. >>> >> >> Yes, that is goal. >> >>>>> >>>> >>>> So far, the only user for the snp_msg_seqno() is the attestation driver. >>>> And the driver is designed to serialize the vmgexit request and thus we >>>> should not run into concurrence issue. >>> >>> That seems a little dangerous as any module new code or out-of-tree >>> module could use this function thus revealing this race condition >>> right? Could we at least have a comment on these functions >>> (snp_msg_seqno and snp_gen_msg_seqno) noting this? >>> >> >> Yes, if the driver is not performing the serialization then we will get >> into race condition. >> >> One way to avoid this requirement is to do all the crypto inside the >> snp_issue_guest_request() and eliminate the need to export the >> snp_msg_seqno(). >> >> I will add the comment about it in the function. > > Actually I forgot that the sequence number is the only component of > the AES-GCM IV. Seen in 'enc_payload'. Given the AES-GCM spec requires > uniqueness of the IV. I think we should try a little harder than a > comment to guarantee we never expose 2 requests encrypted with the > same sequence number / IV. It's more than just a DOS against the > guest's PSP request ability but also could be a guest security issue, > thoughts? > Ah good point, we should avoid a request with same IV. May be move the sequence number increment and save in sevguest drv. Then driver can do the sequence get, vmgexit and increment with a protected lock. thanks > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fnvlpubs.nist.gov%2Fnistpubs%2FLegacy%2FSP%2Fnistspecialpublication800-38d.pdf&data=04%7C01%7Cbrijesh.singh%40amd.com%7C46a05f4713834307706608d973ade616%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637668013461202204%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=KCsi5rTQX6L%2BqY07VdBtF8IH0TLNyHn6wTyidgWvXf4%3D&reserved=0 > (Section 8 page 18) > >> >> thanks >