On Tue, Jan 24, 2023 at 6:38 AM James Bottomley <jejb@xxxxxxxxxxxxx> wrote: > > On Mon, 2023-01-23 at 11:48 -0600, William Roberts wrote: > > On Fri, Jan 20, 2023 at 9:29 PM Jarkko Sakkinen <jarkko@xxxxxxxxxx> > > wrote: > > > > > > On Sat, Jan 14, 2023 at 09:55:37AM -0500, James Bottomley wrote: > > > > On Tue, 2023-01-03 at 13:10 -0800, Matthew Garrett wrote: > > > > > On Tue, Jan 3, 2023 at 1:05 PM William Roberts > > > > > <bill.c.roberts@xxxxxxxxx> wrote: > > > > > > > > > > > What's the use case of using the creation data and ticket in > > > > > > this context? Who gets the creationData and the ticket? > > > > > > Could a user supplied outsideInfo work? IIRC I saw some > > > > > > patches flying around where the sessions will get encrypted > > > > > > and presumably correctly as well. This would allow the > > > > > > transfer of that outsideInfo, like the NV Index PCR value to > > > > > > be included and integrity protected by the session HMAC. > > > > > > > > > > The goal is to ensure that the key was generated by the kernel. > > > > > In the absence of the creation data, an attacker could generate > > > > > a hibernation image using their own key and trick the kernel > > > > > into resuming arbitrary code. We don't have any way to pass > > > > > secret data from the hibernate kernel to the resume kernel, so > > > > > I don't think there's any easy way to do it with outsideinfo. > > > > > > > > Can we go back again to why you can't use locality? It's exactly > > > > designed for this since locality is part of creation data. > > > > Currently everything only uses locality 0, so it's impossible for > > > > anyone on Linux to produce a key with anything other than 0 in > > > > the creation data for locality. However, the dynamic launch > > > > people are proposing that the Kernel should use Locality 2 for > > > > all its operations, which would allow you to distinguish a key > > > > created by the kernel from one created by a user by locality. > > > > > > > > I think the previous objection was that not all TPMs implement > > > > locality, but then not all laptops have TPMs either, so if you > > > > ever come across one which has a TPM but no locality, it's in a > > > > very similar security boat to one which has no TPM. > > > > > > Kernel could try to use locality 2 and use locality 0 as fallback. > > > > I don't think that would work for Matthew, they need something > > reliable to indicate key provenance. > > No, I think it would be good enough: locality 0 means anyone (including > the kernel on a machine which doesn't function correctly) could have > created this key. Locality 2 would mean only the kernel could have > created this key. That's exactly what I was saying, for this feature to be functional 2 localities need to be supported. > > By the time the kernel boots and before it loads the hibernation image > it will know the answer to the question "does my TPM support locality > 2", so it can use that in its security assessment: if the kernel > supports locality 2 and the key wasn't created in locality 2 then > assume an attack. Obviously, if the kernel doesn't support locality 2 > then the hibernation resume has to accept any old key, but that's the > same as the situation today. > Yep, we had this conversation offline on a thread, i'm in agreement here as well. > > I was informed that all 5 localities should be supported starting > > with Gen 7 Kaby Lake launched in 2016. Don't know if this is > > still "too new". > > It's probably good enough. Current laptops which can't use locality 2 > are in the same position as now, but newer ones can provide more > security guarantees. > > There is, however, another wrinkle: can Kaby Lake be persuaded, though > bios settings perhaps, to shut off the non zero localities? I have no idea, and I don't have one handy, but I can ask around. > This would > allow for a downgrade attack where you shut off locality 2 then present > a forged locality 0 key and hibernation image; the kernel will think, > because it can't access locality 2, that it's in a reduced security > environment so the key might be OK. We could fix this by requiring > Kaby Lake and beyond to have locality 2 and refusing to hibernate if it > can't be accessed and building "is this Kaby lake or beyond" into the > check for should I have locality 2, but this is getting complex and > error prone. > > James >
