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

[Peter Gonda]

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
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?

I can do that in this series.

>
> >
> >>>
> >>>>
> >>>> 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
> >
> >
> >