On Thu, Jul 23, 2026 at 04:23:34PM -0700, Carlos Bilbao wrote: > On 7/20/26 02:19, David Hildenbrand (Arm) wrote: > > > > > > We've the virto-mem config struct layout and the kernel source, so for > > > > > obvious fixes like a NULL check, static analysis is better than > > > > > fuzzing. > > > > > Claude took a few mins to find me two examples: > > > > > > > > > > Patch 1: virtio-mem: reject non-power-of-two device_block_size > > > > > This one is for virtio_mem_init() to check if > > > > > !is_power_of_2(vm->device_block_size) > > > > > > > > > > Patch 2: virto-mem: validate region_size and usable_region_size > > > > > THis one checks region_size != 0 and vm->usable_reion_size > > > > > > vm->region_size. > > > > > > > > > > An endless factory of "silly" checks like these are low hanging fruit. > > > > "silly" is the right word. > > > "silly" in what way? > > > > > As in producing "silly" low-hanging fruit patches that don't move the needle > > when it comes to security. > > > > > Seriously, I'm trying to figure out what you all care about here and > > > what exactly the threat model you want this driver to work in, and I'm > > > getting conflicting answers. > > > > > > Either you all do worry about the "device" sending bad data and want to > > > protect from that, or you don't and you trust it. Pick one please so > > > that we know how to deal with these bug reports we are getting. > > > > > > For example, for USB we have said our threat model is: > > > > > > - we do NOT trust the device before a driver is bound to the device, > > > so if a malicious device can do something to the kernel, the kernel > > > needs to be fixed. > > > - During the probe() call for a USB driver, the driver does NOT trust > > > the device, and again, anything a malicious device can do to the > > > kernel, the kernel should fix. > > > - After probe() for a USB driver succeeds, it's up to the driver if it > > > wants to validate all data coming from the device or not. Right > > > now, in general, the kernel trusts the device at that point in time > > > so additional checks are discretionary and at the whim of the > > > maintainer. > > > > > > For that last point, I will note that some BIG users of Linux (i.e. > > > billions of Android devices) still explicitly do NOT want to trust the > > > USB device at this point in time, and are relying on the kernel to > > > protect the system from bad devices. In that case, various patches have > > > been taken to different drivers and subsystems to play whack-a-mole on > > > while Android gets their act together to finally come up with a solid > > > defensive plan (like ChromeOS has had for a decade.) It will be seen > > > which happens first, all drivers are properly fuzzed and fixed up, or > > > Android gets their act together and finally fixes their b0rked system > > > trust model. I think Android management is relying on the kernel > > > community to do the kernel work as they keep refusing to staff the > > > userspace work that they need to do here... > > Right, and for virtio devices trusting the device after probe is just > > extremely > > questionable. > > > > What changes during probe that the device suddenly sends us good data? > > > > Why would a hypervisor that tried to break us before probe not try to break > > us > > after probe? > > > > > And yes, I really need to write this up in a more solid document for USB > > > and get it into the tree, but at least this email thread has forced me > > > to write down the above :) > > > > > > > > > So, again, for virtio drivers, what exactly do you all want to say is > > > your threat model that the drivers need to handle? Can you all agree on > > > something please? Otherwise, for new developers like Hari, this is > > > totaly confusion as to what they should be doing. > > Well, I am also totally confused why we end up checking against some MUST > > clauses in the spec, but not against others. > > > > I am very much in favor of making virtio-mem completely safe to use even in > > coco, where it is currently not used at all. > > > > If it's really about "don't let a device trigger any unexpected kernel code > > execution by sanitizing all input data", fine with me. We should do exactly > > that. Try checking all MUST clauses etc. > > > > But I don't think doing the "low hanging fruit" adds any security. It > > should be > > done properly or not at all. > > > I think you're mistaken in assuming that these "silly" fixes don't move the > needle at all when it comes to security. We can't really quantify these > things, and one extra NULL check by itself is probably not going to make a > meaningful difference. But, IMHO, this should be treated as the opposite of > "death by a thousand cuts", many small hardening improvements, each > individually insignificant, can collectively make the kernel substantially > more robust over long periods of time. > > > Thanks, > > Carlos
Simply put - no. DoS attacks, including NULL ptr accesses, are outside the confidential computing security boundary. They have to be - denying service to whoever is not paying them is exactly how cloud vendors get paid. My preference is to ignore such non-issues - we need to focus on handling real ones. Besides, adding random changes all over the code for non issues can easily introduce new security bugs. That's the classic death by a thousand cuts. Thanks, -- MST
