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Wed, 22 Jul 2026 07:36:08 +0000 Date: Wed, 22 Jul 2026 00:36:06 -0700 From: Peter Fang To: "Edgecombe, Rick P" CC: "Hansen, Dave" , "kvm@vger.kernel.org" , "bp@alien8.de" , "kas@kernel.org" , "Li, Xiaoyao" , "x86@kernel.org" , "sathyanarayanan.kuppuswamy@linux.intel.com" , "mingo@redhat.com" , "dave.hansen@linux.intel.com" , "tglx@kernel.org" , "linux-coco@lists.linux.dev" , "hpa@zytor.com" , "linux-kernel@vger.kernel.org" , "binbin.wu@linux.intel.com" Subject: Re: [PATCH v2 2/2] virt: tdx-guest: Allocate Quote buffer dynamically Message-ID: <20260722073606.GI3178326@pedri> References: <20260717214349.4075994-1-peter.fang@intel.com> <20260717214349.4075994-3-peter.fang@intel.com> <51059852-9a10-4735-a81d-003b4b1963a6@intel.com> <20260721064907.GG3178326@pedri> <741eb8b8b83dd10d39f304133a7625f4232ab793.camel@intel.com> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <741eb8b8b83dd10d39f304133a7625f4232ab793.camel@intel.com> X-ClientProxiedBy: SJ0PR13CA0056.namprd13.prod.outlook.com (2603:10b6:a03:2c2::31) To SJ0PR11MB5645.namprd11.prod.outlook.com (2603:10b6:a03:3b9::19) Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ0PR11MB5645:EE_|PH0PR11MB5805:EE_ X-MS-Office365-Filtering-Correlation-Id: 26a3e021-49f2-4dce-9f7f-08dee7c3e4e9 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|366016|1800799024|7416014|376014|56012099006|11063799006|4143699003|10067099003|22082099003|18002099003; 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Maybe that's not how we do things today, > > > > but if a random dude on the Internet can spew one line to dmesg, is it > > > > that crazy that a bad VMM be able to spew a warning? > > > > > > That makes sense... I actually argued with AI about this but it kept > > > saying this is kind of like DoSing the guest. > > A guest DOSing another guest is relevant, but the host DOSing the guest is not a > threat model we should or even can care about. If userspace or the kernel > decides to kill the guest, that is its prerogative. At most it's a functional > bug and not a security one. Hmm good point. So maybe treat this more like a bug than an attack. Then the direction is clearer. I went back to my debate with AI and found what spooked me: "Letting untrusted input reach a WARN is a well-known anti-pattern precisely because of panic_on_warn — it's why syzkaller treats any WARN as a bug." -> but this is more of a bug than an untrusted input "'Fail hard to defend against the attacking host' buys you zero additional protection, while adding log-spam and a benign-misconfig panic vector." -> but there is also no point in limping along > > But... doesn't this size come from the TDX module? In which case there is no > hope of defense from anything. Yes it's directly from the TDX module. So maybe it's more of a question of how loud this failure should be. There is already a: pr_err("Failed to allocate Quote buffer\n"); ... later if buffer allocation fails, so the user will see something in dmesg. But maybe it's too subtle... > > > > But thinking about it > > > more, tainting the guest is probably the right thing to do... At least > > > the guest sees a big splat about why attestation is failing. I'll remove > > > the __GFP_NOWARN. Thanks. > > > > This does seem like the kind of high-level TDX policy that we all need > > to be aligned on and probably document somewhere. The basic question is > > whether a TDX guest should be quiet or verbose in the face of host > > malfunction or malfeasance. > > > > Kirill and Rick, what do you think? > > We discussed something similar around the set_memory_en/decrypted() failures. > But that was around whether to panic or just warn. Not be silent. If the host is > mucking around, I think security conscious guests would want to know, and even > have the option to pass panic_on_warn. So yea I think we should assume the > default TDX user is security conscious. "Warn on untrusted host weird behavior" > seems like a good policy. Sounds good to me. Thanks for providing more background. > > BUT, is this number coming from the untrusted host or the TDX module? I cant > find TDCS_QUOTE_MAX_SIZE in the docs. If it is coming from the TDX module then > we are not talking about security considerations at all. It's just a functional > "do you want to know if things are failing". Not sure why this one would be > special in that regard. If we think it will fail so often that we don't want a > warning, then we probably need another solution. Another consideration I had at the time was that this happens before attestation. And not being able to do attestation felt like a big enough red flag (I'm assuming something in the guest would scream). So that led me to think that perhaps a warn would be too noisy. But I'm good with following the TDX policy and just warn. I think this tilts the scale towards keeping the warn. Let me know if anyone thinks otherwise. Thanks for the discussion!