Re: [PATCH] virt: Prevent AES-GCM IV reuse in SNP guest driver

[Peter Gonda]

On Wed, Oct 19, 2022 at 2:58 PM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>
> On 10/19/22 15:39, Peter Gonda wrote:
> > On Wed, Oct 19, 2022 at 1:56 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.
> >
> > Ah, I forgot the host could keep changing the size of this data.
> >
> >>
> >>> 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.
> >> 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.
> >>
> >> Peter, is this something you could change the patch to do?
> >
> > OK so the guest retires with the same request when it gets an
> > SNP_GUEST_REQ_INVALID_LEN error. It expands its internal buffer to
>
> It would just use the pre-allocated snp_dev->certs_data buffer with npages
> set to the full size of that buffer.

Actually we allocate that buffer with size SEV_FW_BLOB_MAX_SIZE. Maybe
we want to just allocate this buffer which we think is sufficient and
never increase the allocation?

I see the size of
https://developer.amd.com/wp-content/resources/ask_ark_milan.cert is
3200 bytes. Assuming the VCEK cert is the same size (which it should
be since this .cert is 2 certificates). 16K seems to leave enough room
even for some vendor certificates?

>
> > hold the certificates. When it finally gets a successful request w/
> > certs. Do we want to return the attestation bits to userspace, but
> > leave out the certificate data. Or just error out the ioctl
> > completely?
>
> We need to be able to return the attestation bits that came back with the
> extra certs. So just error out of the ioctl with the length error and let
> user-space retry with the recommended number of pages.

That sounded simpler to me. Will do.

>
> >
> > I can do that in this series.
>
> Thanks!
>
> >
> >>
> >>>
> >>>>>
> >>>>>>
> >>>>>> 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?
> >>
> >>>
> >>> Calling the retry ioctl without a pending command will result in -EINVAL.
> >>>
> >>> Let me know what you think.
> >>
> >> 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.
> >
> > Whats the issue with the guest driver taking some time?
> >
> > This sounds complex because there may be many users of the driver. How
> > do multiple users coordinate when they need to use the retry ioctl?
> >
> >>
> >> Thanks,
> >> Tom
> >>
> >>>>
> >>>> Thanks,
> >>>> Tom
> >>>
> >>>
> >>>