Messages in this thread | | | Date | Wed, 19 Oct 2022 14:56:22 -0500 | Subject | Re: [PATCH] virt: Prevent AES-GCM IV reuse in SNP guest driver | From | Tom Lendacky <> |
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On 10/19/22 14:17, Dionna Amalie Glaze wrote: > On Wed, Oct 19, 2022 at 11:44 AM Tom Lendacky <thomas.lendacky@amd.com> wrote: >> >> On 10/19/22 12:40, Peter Gonda wrote: >>> On Wed, Oct 19, 2022 at 11:03 AM Tom Lendacky <thomas.lendacky@amd.com> 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. 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?
> >>> >>>> >>>> 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@google.com/ >>> >>> 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.
Thanks, Tom
>> >> Thanks, >> Tom > > >
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