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

From: Tom Lendacky
Date: Wed Oct 19 2022 - 16:58:46 EST

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.

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.


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