Re: [PATCH RFC v7 62/64] x86/sev: Add KVM commands for instance certs

[Dov Murik]

Hi Peter,

On 10/01/2023 17:23, Peter Gonda wrote:
> On Tue, Jan 10, 2023 at 8:10 AM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>>
>> On 1/10/23 01:10, Dov Murik wrote:
>>> Hi Tom,
>>>
>>> On 10/01/2023 0:27, Tom Lendacky wrote:
>>>> On 1/9/23 10:55, Dionna Amalie Glaze wrote:
>>>>>>> +
>>>>>>> +static int snp_set_instance_certs(struct kvm *kvm, struct
>>>>>>> kvm_sev_cmd *argp)
>>>>>>> +{
>>>>>> [...]
>>>>>>
>>>>>> Here we set the length to the page-aligned value, but we copy only
>>>>>> params.cert_len bytes.  If there are two subsequent
>>>>>> snp_set_instance_certs() calls where the second one has a shorter
>>>>>> length, we might "keep" some leftover bytes from the first call.
>>>>>>
>>>>>> Consider:
>>>>>> 1. snp_set_instance_certs(certs_addr point to "AAA...", certs_len=8192)
>>>>>> 2. snp_set_instance_certs(certs_addr point to "BBB...", certs_len=4097)
>>>>>>
>>>>>> If I understand correctly, on the second call we'll copy 4097 "BBB..."
>>>>>> bytes into the to_certs buffer, but length will be (4096 + PAGE_SIZE -
>>>>>> 1) & PAGE_MASK which will be 8192.
>>>>>>
>>>>>> Later when fetching the certs (for the extended report or in
>>>>>> snp_get_instance_certs()) the user will get a buffer of 8192 bytes
>>>>>> filled with 4097 BBBs and 4095 leftover AAAs.
>>>>>>
>>>>>> Maybe zero sev->snp_certs_data entirely before writing to it?
>>>>>>
>>>>>
>>>>> Yes, I agree it should be zeroed, at least if the previous length is
>>>>> greater than the new length. Good catch.
>>>>>
>>>>>
>>>>>> Related question (not only for this patch) regarding snp_certs_data
>>>>>> (host or per-instance): why is its size page-aligned at all? why is it
>>>>>> limited by 16KB or 20KB? If I understand correctly, for SNP, this buffer
>>>>>> is never sent to the PSP.
>>>>>>
>>>>>
>>>>> The buffer is meant to be copied into the guest driver following the
>>>>> GHCB extended guest request protocol. The data to copy back are
>>>>> expected to be in 4K page granularity.
>>>>
>>>> I don't think the data has to be in 4K page granularity. Why do you
>>>> think it does?
>>>>
>>>
>>> I looked at AMD publication 56421 SEV-ES Guest-Hypervisor Communication
>>> Block Standardization (July 2022), page 37.  The table says:
>>>
>>> --------------
>>>
>>> NAE Event: SNP Extended Guest Request
>>>
>>> Notes:
>>>
>>> RAX will have the guest physical address of the page(s) to hold returned
>>> data
>>>
>>> RBX
>>> State to Hypervisor: will contain the number of guest contiguous
>>> pages supplied to hold returned data
>>> State from Hypervisor: on error will contain the number of guest
>>> contiguous pages required to hold the data to be returned
>>>
>>> ...
>>>
>>> The request page, response page and data page(s) must be assigned to the
>>> hypervisor (shared).
>>>
>>> --------------
>>>
>>>
>>> According to this spec, it looks like the sizes are communicated as
>>> number of pages in RBX.  So the data should start at a 4KB alignment
>>> (this is verified in snp_handle_ext_guest_request()) and its length
>>> should be 4KB-aligned, as Dionna noted.
>>
>> That only indicates how many pages are required to hold the data, but the
>> hypervisor only has to copy however much data is present. If the data is
>> 20 bytes, then you only have to copy 20 bytes. If the user supplied 0 for
>> the number of pages, then the code returns 1 in RBX to indicate that one
>> page is required to hold the 20 bytes.
>>
>>>
>>> I see no reason (in the spec and in the kernel code) for the data length
>>> to be limited to 16KB (SEV_FW_BLOB_MAX_SIZE) but I might be missing some
>>> flow because Dionna ran into this limit.
>>
>> Correct, there is no limit. I believe that SEV_FW_BLOB_MAX_SIZE is a way
>> to keep the memory usage controlled because data is coming from userspace
>> and it isn't expected that the data would be larger than that.
>>
>> I'm not sure if that was in from the start or as a result of a review
>> comment. Not sure what is the best approach is.
> 
> This was discussed a bit in the guest driver changes recently too that
> SEV_FW_BLOB_MAX_SIZE is used in the guest driver code for the max cert
> length. We discussed increasing the limit there after fixing the IV
> reuse issue.

I see it now.

(Joerg, maybe we should add F:drivers/virt/coco/ to the MAINTAINERS list
so that patches there are hopefully sent to linux-coco?)


> 
> Maybe we could introduce SEV_CERT_BLOB_MAX_SIZE here to be more clear
> there is no firmware based limit? Then we could switch the guest
> driver to use that too. Dionna confirmed 4 pages is enough for our
> current usecase, Dov would you recommend something larger to start?
> 

Introducing a new constant sounds good to me (and use the same constant
in the guest driver).

I think 4 pages are OK; I also don't see real harm in increasing this
limit to 1 MB (if the host+guest agree to pass more stuff there, besides
certificates).  But maybe that's just abusing this channel, and for
other data we should use other mechanisms (like vsock).

-Dov