On Wed, Oct 19, 2022 at 12:56 PM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote: > > On 10/19/22 14:17, Dionna Amalie Glaze wrote: > > On Wed, Oct 19, 2022 at 11:44 AM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote: > >> > >> On 10/19/22 12:40, Peter Gonda wrote: > >>> On Wed, Oct 19, 2022 at 11:03 AM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote: > >>>> > >>>> On 10/19/22 10:03, Peter Gonda wrote: > >>>>> The ASP and an SNP guest use a series of AES-GCM keys called VMPCKs to > >>>>> communicate securely with each other. The IV to this scheme is a > >>>>> sequence number that both the ASP and the guest track. Currently this > >>>>> sequence number in a guest request must exactly match the sequence > >>>>> number tracked by the ASP. This means that if the guest sees an error > >>>>> from the host during a request it can only retry that exact request or > >>>>> disable the VMPCK to prevent an IV reuse. AES-GCM cannot tolerate IV > >>>>> reuse see: > >>>>> https://csrc.nist.gov/csrc/media/projects/block-cipher-techniques/documents/bcm/comments/800-38-series-drafts/gcm/joux_comments.pdf > >>>> > >> > >> I think I've wrapped my head around this now. Any non-zero return code > >> from the hypervisor for an SNP Guest Request is either a hypervisor error > >> or an sev-guest driver error, and so the VMPCK should be disabled. The > >> sev-guest driver is really doing everything (message headers, performing > >> the encryption, etc.) and is only using userspace data that will be part > >> of the response message and can't result in a non-zero hypervisor return code. > >> > >> For the SNP Extended Guest Request, we only need to special case a return > >> code of SNP_GUEST_REQ_INVALID_LEN. See below for my responses on that. > >> > >> > >>>> I wonder if we can at least still support the extended report length query > >>>> by having the kernel allocate the required pages when the error is > >>>> SNP_GUEST_REQ_INVALID_LEN and retry the exact request again. If there are > >>>> no errors on the second request, the sequence numbers can be safely > >>>> updated, but the kernel returns the original error (which will provide the > >>>> caller with the number of pages required). > >>> > >>> I think we can but I thought fixing the security bug could come first, > >>> then the usability fix after. Dionna was planning on working on that > >>> fix. > >>> > >>> In that flow how does userspace get the data? Its called the ioctl > >>> with not enough output buffer space. What if the userspace calls the > >>> ioctl with no buffers space allocated, so its trying to query the > >>> length. We just send the host the request without any encrypted data. > >> > >> In the case of SNP_GUEST_REQ_INVALID_LEN, userspace wouldn't get the data > >> if it hasn't supplied enough buffer space. But, the sev-guest driver can > >> supply enough buffer space and invoke the SNP Extended Guest Request again > >> in order to successfully complete the call and update the sequence > >> numbers. The sev-guest driver would just discard the data in this case, > >> but pass back the original "not enough buffer space" error to the caller, > >> who could now allocate space and retry. This then allows the sequence > >> numbers to be bumped properly. > >> > > > > The way I thought to solve this was to make certificate length > > querying a part of the specified protocol. > > > > The first ext_guest_request command /must/ query the certificate > > buffer length with req.certs_len == 0. > > This becomes an incompatible change to the GHCB specification. > > > By making this part of the protocol, the sev-guest driver can check if > > the certificate length has been requested before. > > If so, emulate the host's VMM error code for invalid length without > > sending an encrypted message. > > On the hypervisor side, the certificate blob can be replaced at any time > with a new blob that is larger. So you may still have to handle the case > where you get a SNP_GUEST_REQ_INVALID_LEN even if you previously asked before. > > > If not, then send an all zeroes request buffer with the req.certs_len > > = 0 values to the VMM. > > > > The VMM will respond with the size if indeed the expected_pages are > > > 0. In the case that the host has not set the certificate buffer yet, > > then the host will inspect the header of the request page for a zero > > sequence number. If so, then we know that we don't have a valid > > request. We treat this also as the INVALID_LEN case but still return > > the size of 0. The driver will have the expected pages value stored as > > 0 at this point, so subsequent calls will not have this behavior. > > > > The way /dev/sev-guest user code has been written, I don't think this > > will break any existing software package. > > I think having the sev-guest driver re-issue the request with the internal > buffer when it receives SNP_GUEST_REQ_INVALID_LEN is the better way to go. I take it you mean in the case that the host's certs_len == 0? > You could still cache the size request and always return that to > user-space when a request is received with a 0 length. The user-space > program must be able to handle receiving multiple > SNP_GUEST_REQ_INVALID_LEN in succession anyway, because of the fact that > the hypervisor can be updating the certs asynchronously. And if you get a > request that is not 0 length, then you issue it as such and re-use the > logic of the first 0 length request that was received if you get an > SNP_GUEST_REQ_INVALID_LEN with the user-space supplied value. > A request that gets SNP_GUEST_REQ_INVALID_LEN when the guest expects that it is providing a sufficiently sized certificate buffer means that the guest has encrypted its report request. We then have a harder problem than throttling because not only do we have to reissue the same request, it must be with different certificate arguments provided from user space. > Peter, is this something you could change the patch to do? > > > > >>> > >>>> > >>>> For the rate-limiting patch series [1], the rate-limiting will have to be > >>>> performed within the kernel, while the mutex is held, and then retry the > >>>> exact request again. Otherwise, that error will require disabling the > >>>> VMPCK. Either that, or the hypervisor must provide the rate limiting. > >>>> > >>>> Thoughts? > >>>> > >>>> [1] https://lore.kernel.org/lkml/20221013160040.2858732-1-dionnaglaze@xxxxxxxxxx/ > >>> > >>> Yes I think if the host rate limits the guest. The guest kernel should > >>> retry the exact message. Which mutex are you referring too? > >> > >> Or the host waits and then submits the request and the guest kernel > >> doesn't have to do anything. The mutex I'm referring to is the > >> snp_cmd_mutex that is taken in snp_guest_ioctl(). > > > > I think that either the host kernel or guest kernel waiting can lead > > to unacceptable delays. > > I would recommend that we add a zero argument ioctl to /dev/sev-guest > > specifically for retrying the last request. > > > > We can know what the last request is due to the sev_cmd_mutex serialization. > > The driver will just keep a scratch buffer for this. Any other request > > that comes in without resolving the retry will get an -EBUSY error > > code. > > And the first caller will have received an -EAGAIN in order to > differentiate between the two situations? Yes, the throttled caller gets -EAGAIN, and other ioctls other than retry after that get -EBUSY. > > I think that sounds reasonable, but there are some catches. You will need > to ensure that the caller that is supposed to retry does actually retry > and that a caller that does retry is the same caller that was told to retry. > I think that constitutes a change to task_struct, the way that there's a buffer for interrupted system calls. That seems a bit much. Do we have to model for protocol-breaking user tasks that have access to /dev/sev-guest? The caller that gets -EAGAIN knows to retry. There's no reason for other tasks to retry due to command serialization and the -EBUSY behavior. -- -Dionna Glaze, PhD (she/her)
