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

[Kalra, Ashish]

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)

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) {
                *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);
> > +}
> > +