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[80.230.24.117]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4954a2edbe4sm39919225e9.12.2026.07.17.03.52.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 17 Jul 2026 03:52:49 -0700 (PDT) Date: Fri, 17 Jul 2026 06:52:46 -0400 From: "Michael S. Tsirkin" To: Greg Kroah-Hartman Cc: "David Hildenbrand (Arm)" , Hari Mishal , Jason Wang , Xuan Zhuo , Eugenio =?iso-8859-1?Q?P=E9rez?= , virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, elena.reshetova@intel.com, carlos.bilbao.osdev@gmail.com Subject: Re: [PATCH v2 1/4] virtio-mem: validate device-reported block size Message-ID: <20260717065219-mutt-send-email-mst@kernel.org> References: <2026071635-relive-flogging-2a81@gregkh> <4dda47ba-534a-4297-a25e-0d63d9167033@kernel.org> <20260717014134-mutt-send-email-mst@kernel.org> <3b32a38f-0964-45b9-9529-933abedbf69b@kernel.org> <20260717044019-mutt-send-email-mst@kernel.org> <2026071746-deviation-clad-1712@gregkh> <20260717060822-mutt-send-email-mst@kernel.org> <2026071757-grout-composer-165d@gregkh> <20260717061901-mutt-send-email-mst@kernel.org> <2026071724-asleep-pedigree-ea54@gregkh> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <2026071724-asleep-pedigree-ea54@gregkh> On Fri, Jul 17, 2026 at 12:46:52PM +0200, Greg Kroah-Hartman wrote: > On Fri, Jul 17, 2026 at 06:23:57AM -0400, Michael S. Tsirkin wrote: > > On Fri, Jul 17, 2026 at 12:15:09PM +0200, Greg Kroah-Hartman wrote: > > > On Fri, Jul 17, 2026 at 06:10:41AM -0400, Michael S. Tsirkin wrote: > > > > On Fri, Jul 17, 2026 at 11:14:23AM +0200, Greg Kroah-Hartman wrote: > > > > > On Fri, Jul 17, 2026 at 04:59:32AM -0400, Michael S. Tsirkin wrote: > > > > > > On Fri, Jul 17, 2026 at 10:39:40AM +0200, David Hildenbrand (Arm) wrote: > > > > > > > On 7/17/26 07:48, Michael S. Tsirkin wrote: > > > > > > > > On Thu, Jul 16, 2026 at 05:59:05PM +0200, David Hildenbrand (Arm) wrote: > > > > > > > >>> Or do we just always trust virtio mem devices explicitly? > > > > > > > >> > > > > > > > >> It's hard for me to understand where we draw the line, really. > > > > > > > >> > > > > > > > >> But maybe MST can clarify what we care about in virtio world where the > > > > > > > >> hypervisor is fully in charge of the device, > > > > > > > > > > > > > > > > Generally: > > > > > > > > - The guest is expected to whitelist drivers (most drivers have not > > > > > > > > been audited). > > > > > > > > > > > > > > But even if you audited your driver, who makes sure that we consider all ways > > > > > > > where the device could mess with us? > > > > > > > > > > > > A lot of this is up to a correct setup. For example, make sure all > > > > > > filesystems are encrypted and refuse to mount unencrypted ones. > > > > > > > > > > > > > Something feels off here. > > > > > > > > > > > > > > Handling selected out-of-spec scenarios like this feels like a band-aid. Happy > > > > > > > to be corrected. > > > > > > > > > > > > Well Documentation/security/snp-tdx-threat-model.rst puts it like this: > > > > > > It is important to note > > > > > > that this doesn’t imply that the host or VMM are intentionally > > > > > > malicious, but that there exists a security value in having a small CoCo > > > > > > VM TCB. > > > > > > > > > > > > and > > > > > > > > > > > > While traditionally the host has unlimited access to guest data and can > > > > > > leverage this access to attack the guest, the CoCo systems mitigate such > > > > > > attacks by adding security features like guest data confidentiality and > > > > > > integrity protection. > > > > > > > > > > > > > > > > > > now, when we are talking about "mitigation" it is indeed becoming a bit > > > > > > murky. > > > > > > > > > > > > > > > > > > For me, a rule of thumb I came up with is that if the validation happens > > > > > > to also be helful for users e.g. to work around buggy devices, > > > > > > or maybe because we feel failing gracefully is nice because this > > > > > > will allow to later make use of this config and old drivers will > > > > > > fail but at least not panic, then it is good to include. > > > > > > > > > > Why not do what USB does? Don't trust the device until AFTER probe() > > > > > succeeds? All of the needed checking should happen before then, as that > > > > > is a "slow path" so lots of validation and the like can happen at that > > > > > point. > > > > > > > > > > After that, during the normal data paths, after the driver is bound, > > > > > trust it all you want as attempting to validate every single packet is > > > > > just going to be impossible. > > > > > > > > > > thanks, > > > > > > > > > > greg k-h > > > > > > > > People do expect that data path validation at this point. > > > > > > Ok, so you want this patch :) > > > > > > And more, as you need to treat everything from the host as "untrusted", > > > and it must be "verified". > > > > Well. First it's not me) Second it's only specific configurations - > > for example there's no short term plan to validate filesystem code, people > > are expected to rely on encryption. The reasons have more to do > > with the available manpower than anything else. > > Sure, but again, for subsystems, you have to define your threat model as > the LLMs are churning against the code base and coming up with lots of > crazy ideas if a device should or should not be trusted and spitting out > patches and reports like the ones that are in the first few patches of > this series. > > So please, pick a model, let's document it, and go with that. I am > getting directly conflicting responses here. > > thanks, > > greg k-h Supposed to be this one: Documentation/security/snp-tdx-threat-model.rst what is missing? -- MST