Re: [PATCH Part2 v6 42/49] KVM: SVM: Provide support for SNP_GUEST_REQUEST NAE event

Kalra, Ashish
2022-10-21T21:56:22Z

On 10/21/2022 4:30 PM, Tom Lendacky wrote:
> On 10/21/22 16:12, Kalra, Ashish wrote:
>> Hello Tom,
>>
>> On 10/21/2022 2:06 PM, Tom Lendacky wrote:
>>> On 6/20/22 18:13, Ashish Kalra wrote:
>>>> From: Brijesh Singh <brijesh.singh@amd.com>
>>>>
>>>> Version 2 of GHCB specification added the support for two SNP Guest
>>>> Request Message NAE events. The events allows for an SEV-SNP guest to
>>>> make request to the SEV-SNP firmware through hypervisor using the
>>>> SNP_GUEST_REQUEST API define in the SEV-SNP firmware specification.
>>>>
>>>> The SNP_EXT_GUEST_REQUEST is similar to SNP_GUEST_REQUEST with the
>>>> difference of an additional certificate blob that can be passed through
>>>> the SNP_SET_CONFIG ioctl defined in the CCP driver. The CCP driver
>>>> provides snp_guest_ext_guest_request() that is used by the KVM to get
>>>> both the report and certificate data at once.
>>>>
>>>> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
>>>> ---
>>>>   arch/x86/kvm/svm/sev.c | 196
>>>> +++++++++++++++++++++++++++++++++++++++--
>>>>   arch/x86/kvm/svm/svm.h |   2 +
>>>>   2 files changed, 192 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>>>> index 7fc0fad87054..089af21a4efe 100644
>>>> --- a/arch/x86/kvm/svm/sev.c
>>>> +++ b/arch/x86/kvm/svm/sev.c
>>>
>>>> +static void snp_handle_ext_guest_request(struct vcpu_svm *svm,
>>>> gpa_t req_gpa, gpa_t resp_gpa)
>>>> +{
>>>> +    struct sev_data_snp_guest_request req = {0};
>>>> +    struct kvm_vcpu *vcpu = &svm->vcpu;
>>>> +    struct kvm *kvm = vcpu->kvm;
>>>> +    unsigned long data_npages;
>>>> +    struct kvm_sev_info *sev;
>>>> +    unsigned long rc, err;
>>>> +    u64 data_gpa;
>>>> +
>>>> +    if (!sev_snp_guest(vcpu->kvm)) {
>>>> +        rc = SEV_RET_INVALID_GUEST;
>>>> +        goto e_fail;
>>>> +    }
>>>> +
>>>> +    sev = &to_kvm_svm(kvm)->sev_info;
>>>> +
>>>> +    data_gpa = vcpu->arch.regs[VCPU_REGS_RAX];
>>>> +    data_npages = vcpu->arch.regs[VCPU_REGS_RBX];
>>>> +
>>>> +    if (!IS_ALIGNED(data_gpa, PAGE_SIZE)) {
>>>> +        rc = SEV_RET_INVALID_ADDRESS;
>>>> +        goto e_fail;
>>>> +    }
>>>> +
>>>> +    /* Verify that requested blob will fit in certificate buffer */
>>>> +    if ((data_npages << PAGE_SHIFT) > SEV_FW_BLOB_MAX_SIZE) {
>>>
>>> Not sure this is a valid check...  Isn't it OK if the guest has
>>> supplied more room than is required? If the guest supplies 8 pages
>>> and the hypervisor only needs to copy 1 page of data (or the
>>> SEV_FW_BLOB_MAX_SIZE number of pages) that shouldn't be an error. I
>>> think this check can go, right?
>>>
>>
>> Agreed.
>>
>> The check should probably be
>>   if ((data_npages << PAGE_SHIFT) < SEV_FW_BLOB_MAX_SIZE)
>
> No, the check should just be removed. If the number of pages required to
> hold the cert data is only 1, then a data_npages value of 1 is just fine
> (see below).
>

Ok.

>>
>> and that check already exists in:
>>
>> snp_guest_ext_guest_request(...)
>> {
>> ...
>> ...
>>     /*
>>           * Check if there is enough space to copy the certificate
>> chain. Otherwise
>>           * return ERROR code defined in the GHCB specification.
>>           */
>>          expected_npages = sev->snp_certs_len >> PAGE_SHIFT;
>>          if (*npages < expected_npages) {
>
> If expected_npages is 1, then an *npages value of 1 is OK. But if you
> put the check in above that you want, you would never get here with an
> *npages value of 1.

Yes that is correct, i incorrectly assumed that
sev->snp_certs_len == SEV_FW_BLOB_MAX_SIZE

Thanks,
Ashish

>
> Thanks,
> Tom
>
>>                  *npages = expected_npages;
>>                  *fw_err = SNP_GUEST_REQ_INVALID_LEN;
>>                  return -EINVAL;
>>          }
>> ...
>>
>> Thanks,
>> Ashish
>>
>>> Thanks,
>>> Tom
>>>
>>>> +        rc = SEV_RET_INVALID_PARAM;
>>>> +        goto e_fail;
>>>> +    }
>>>> +
>>>> +    mutex_lock(&sev->guest_req_lock);
>>>> +
>>>> +    rc = snp_setup_guest_buf(svm, &req, req_gpa, resp_gpa);
>>>> +    if (rc)
>>>> +        goto unlock;
>>>> +
>>>> +    rc = snp_guest_ext_guest_request(&req, (unsigned
>>>> long)sev->snp_certs_data,
>>>> +                     &data_npages, &err);
>>>> +    if (rc) {
>>>> +        /*
>>>> +         * If buffer length is small then return the expected
>>>> +         * length in rbx.
>>>> +         */
>>>> +        if (err == SNP_GUEST_REQ_INVALID_LEN)
>>>> +            vcpu->arch.regs[VCPU_REGS_RBX] = data_npages;
>>>> +
>>>> +        /* pass the firmware error code */
>>>> +        rc = err;
>>>> +        goto cleanup;
>>>> +    }
>>>> +
>>>> +    /* Copy the certificate blob in the guest memory */
>>>> +    if (data_npages &&
>>>> +        kvm_write_guest(kvm, data_gpa, sev->snp_certs_data,
>>>> data_npages << PAGE_SHIFT))
>>>> +        rc = SEV_RET_INVALID_ADDRESS;
>>>> +
>>>> +cleanup:
>>>> +    snp_cleanup_guest_buf(&req, &rc);
>>>> +
>>>> +unlock:
>>>> +    mutex_unlock(&sev->guest_req_lock);
>>>> +
>>>> +e_fail:
>>>> +    svm_set_ghcb_sw_exit_info_2(vcpu, rc);
>>>> +}
>>>> +