Re: [PATCH RFC v8 54/56] x86/sev: Add KVM commands for instance certs

[Dov Murik]

Hi Mike,

On 20/02/2023 20:38, Michael Roth wrote:
> From: Dionna Glaze <dionnaglaze@xxxxxxxxxx>
> 
> The /dev/sev device has the ability to store host-wide certificates for
> the key used by the AMD-SP for SEV-SNP attestation report signing,
> but for hosts that want to specify additional certificates that are
> specific to the image launched in a VM, a different way is needed to
> communicate those certificates.
> 
> Add two new KVM ioctl to handle this: KVM_SEV_SNP_{GET,SET}_CERTS
> 
> The certificates that are set with this command are expected to follow
> the same format as the host certificates, but that format is opaque
> to the kernel.
> 
> The new behavior for custom certificates is that the extended guest
> request command will now return the overridden certificates if they
> were installed for the instance. The error condition for a too small
> data buffer is changed to return the overridden certificate data size
> if there is an overridden certificate set installed.
> 
> Setting a 0 length certificate returns the system state to only return
> the host certificates on an extended guest request.
> 
> Also increase the SEV_FW_BLOB_MAX_SIZE another 4K page to allow space
> for an extra certificate.
> 
> Cc: Tom Lendacky <Thomas.Lendacky@xxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> 
> Signed-off-by: Dionna Glaze <dionnaglaze@xxxxxxxxxx>
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> [mdr: remove used of "we" and "this patch" in commit log]
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> ---
>  arch/x86/kvm/svm/sev.c   | 111 ++++++++++++++++++++++++++++++++++++++-
>  arch/x86/kvm/svm/svm.h   |   1 +
>  include/linux/psp-sev.h  |   2 +-
>  include/uapi/linux/kvm.h |  12 +++++
>  4 files changed, 123 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 70d5650d8d95..18b64b7005e7 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2089,6 +2089,7 @@ static void *snp_context_create(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  		goto e_free;
>  
>  	sev->snp_certs_data = certs_data;
> +	sev->snp_certs_len = 0;
>  
>  	return context;
>  
> @@ -2404,6 +2405,86 @@ static int snp_launch_finish(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  	return ret;
>  }
>  
> +static int snp_get_instance_certs(struct kvm *kvm, struct kvm_sev_cmd *argp)
> +{
> +	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> +	struct kvm_sev_snp_get_certs params;
> +
> +	if (!sev_snp_guest(kvm))
> +		return -ENOTTY;
> +
> +	if (!sev->snp_context)
> +		return -EINVAL;
> +
> +	if (copy_from_user(&params, (void __user *)(uintptr_t)argp->data,
> +			   sizeof(params)))
> +		return -EFAULT;
> +
> +	/* No instance certs set. */
> +	if (!sev->snp_certs_len)
> +		return -ENOENT;
> +
> +	if (params.certs_len < sev->snp_certs_len) {
> +		/* Output buffer too small. Return the required size. */
> +		params.certs_len = sev->snp_certs_len;
> +
> +		if (copy_to_user((void __user *)(uintptr_t)argp->data, &params,
> +				 sizeof(params)))
> +			return -EFAULT;
> +
> +		return -EINVAL;
> +	}
> +
> +	if (copy_to_user((void __user *)(uintptr_t)params.certs_uaddr,
> +			 sev->snp_certs_data, sev->snp_certs_len))
> +		return -EFAULT;
> +
> +	return 0;
> +}
> +
> +static int snp_set_instance_certs(struct kvm *kvm, struct kvm_sev_cmd *argp)
> +{
> +	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> +	unsigned long length = SEV_FW_BLOB_MAX_SIZE;
> +	void *to_certs = sev->snp_certs_data;
> +	struct kvm_sev_snp_set_certs params;
> +
> +	if (!sev_snp_guest(kvm))
> +		return -ENOTTY;
> +
> +	if (!sev->snp_context)
> +		return -EINVAL;
> +
> +	if (copy_from_user(&params, (void __user *)(uintptr_t)argp->data,
> +			   sizeof(params)))
> +		return -EFAULT;
> +
> +	if (params.certs_len > SEV_FW_BLOB_MAX_SIZE)
> +		return -EINVAL;
> +
> +	/*
> +	 * Setting a length of 0 is the same as "uninstalling" instance-
> +	 * specific certificates.
> +	 */
> +	if (params.certs_len == 0) {
> +		sev->snp_certs_len = 0;
> +		return 0;
> +	}
> +
> +	/* Page-align the length */
> +	length = (params.certs_len + PAGE_SIZE - 1) & PAGE_MASK;
> +

In comments on v7 [1] Dionna agreed adding cleanup here:

  /* The size could shrink and leave garbage at the end. */
  memset(sev->snp_certs_data, 0, SEV_FW_BLOB_MAX_SIZE);

(we can use 'to_certs' in the first argument)

but it wasn't added in v8.

-Dov

[1] https://lore.kernel.org/linux-coco/CAAH4kHYOtzgqSTZQFcRiZwPLCkLAThjsCMdjUCdsBTiP=W0Vxw@xxxxxxxxxxxxxx/


> +	if (copy_from_user(to_certs,
> +			   (void __user *)(uintptr_t)params.certs_uaddr,
> +			   params.certs_len)) {
> +		return -EFAULT;
> +	}
> +
> +	sev->snp_certs_len = length;
> +
> +	return 0;
> +}
> +
>  int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp)
>  {
>  	struct kvm_sev_cmd sev_cmd;
> @@ -2503,6 +2584,12 @@ int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp)
>  	case KVM_SEV_SNP_LAUNCH_FINISH:
>  		r = snp_launch_finish(kvm, &sev_cmd);
>  		break;
> +	case KVM_SEV_SNP_GET_CERTS:
> +		r = snp_get_instance_certs(kvm, &sev_cmd);
> +		break;
> +	case KVM_SEV_SNP_SET_CERTS:
> +		r = snp_set_instance_certs(kvm, &sev_cmd);
> +		break;
>  	default:
>  		r = -EINVAL;
>  		goto out;
> @@ -3550,8 +3637,28 @@ static void snp_handle_ext_guest_request(struct vcpu_svm *svm, gpa_t req_gpa, gp
>  	if (rc)
>  		goto unlock;
>  
> -	rc = snp_guest_ext_guest_request(&req, (unsigned long)sev->snp_certs_data,
> -					 &data_npages, &err);
> +	/*
> +	 * If the VMM has overridden the certs, then change the error message
> +	 * if the size is inappropriate for the override. Otherwise, use a
> +	 * regular guest request and copy back the instance certs.
> +	 */
> +	if (sev->snp_certs_len) {
> +		if ((data_npages << PAGE_SHIFT) < sev->snp_certs_len) {
> +			rc = -EINVAL;
> +			err = SNP_GUEST_REQ_INVALID_LEN;
> +			goto datalen;
> +		}
> +		rc = sev_issue_cmd(kvm, SEV_CMD_SNP_GUEST_REQUEST, &req,
> +				   (int *)&err);
> +	} else {
> +		rc = snp_guest_ext_guest_request(&req,
> +						 (unsigned long)sev->snp_certs_data,
> +						 &data_npages, &err);
> +	}
> +datalen:
> +	if (sev->snp_certs_len)
> +		data_npages = sev->snp_certs_len >> PAGE_SHIFT;
> +
>  	if (rc) {
>  		/*
>  		 * If buffer length is small then return the expected
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 221b38d3c845..dced46559508 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -94,6 +94,7 @@ struct kvm_sev_info {
>  	u64 snp_init_flags;
>  	void *snp_context;      /* SNP guest context page */
>  	void *snp_certs_data;
> +	unsigned int snp_certs_len; /* Size of instance override for certs */
>  	struct mutex guest_req_lock; /* Lock for guest request handling */
>  
>  	u64 sev_features;	/* Features set at VMSA creation */
> diff --git a/include/linux/psp-sev.h b/include/linux/psp-sev.h
> index 92116e2b74fd..3b28b78938f6 100644
> --- a/include/linux/psp-sev.h
> +++ b/include/linux/psp-sev.h
> @@ -22,7 +22,7 @@
>  #define __psp_pa(x)	__pa(x)
>  #endif
>  
> -#define SEV_FW_BLOB_MAX_SIZE	0x4000	/* 16KB */
> +#define SEV_FW_BLOB_MAX_SIZE	0x5000	/* 20KB */
>  
>  /**
>   * SEV platform state
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 6e684bf5f723..ad7e24e43547 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1928,6 +1928,8 @@ enum sev_cmd_id {
>  	KVM_SEV_SNP_LAUNCH_START,
>  	KVM_SEV_SNP_LAUNCH_UPDATE,
>  	KVM_SEV_SNP_LAUNCH_FINISH,
> +	KVM_SEV_SNP_GET_CERTS,
> +	KVM_SEV_SNP_SET_CERTS,
>  
>  	KVM_SEV_NR_MAX,
>  };
> @@ -2075,6 +2077,16 @@ struct kvm_sev_snp_launch_finish {
>  	__u8 pad[6];
>  };
>  
> +struct kvm_sev_snp_get_certs {
> +	__u64 certs_uaddr;
> +	__u64 certs_len;
> +};
> +
> +struct kvm_sev_snp_set_certs {
> +	__u64 certs_uaddr;
> +	__u64 certs_len;
> +};
> +
>  #define KVM_DEV_ASSIGN_ENABLE_IOMMU	(1 << 0)
>  #define KVM_DEV_ASSIGN_PCI_2_3		(1 << 1)
>  #define KVM_DEV_ASSIGN_MASK_INTX	(1 << 2)