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

[Tom Lendacky]

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@xxxxxxx>
> > >
> > > 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@xxxxxxx>
> > > ---
> > >   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).

> 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.

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);
> > > +}
> > > +