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

[Dov Murik]

Hi Dionna, Mike,

On 14/12/2022 21:40, 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.
> 
> This patch adds two new KVM ioctl commands: 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.
> 
> We 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>
> 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 4de952d1d446..d0e58cffd1ed 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2081,6 +2081,7 @@ 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;
>  
> @@ -2364,6 +2365,86 @@ 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;
> +
> +	if (copy_from_user(to_certs,
> +			   (void __user *)(uintptr_t)params.certs_uaddr,
> +			   params.certs_len)) {
> +		return -EFAULT;
> +	}
> +
> +	sev->snp_certs_len = length;

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?

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.

> +
> +	return 0;
> +}
> +
>  int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp)
>  {
>  	struct kvm_sev_cmd sev_cmd;

[...]

> diff --git a/include/linux/psp-sev.h b/include/linux/psp-sev.h
> index a1e6624540f3..970a9de0ed20 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 */
>  

This has effects in drivers/crypto/ccp/sev-dev.c
                                                               (for
example in alloc_snp_host_map).  Is that OK?


-Dov

>  /**
>   * SEV platform state
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 61b1e26ced01..48bcc59cf86b 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1949,6 +1949,8 @@ 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,
>  };
> @@ -2096,6 +2098,16 @@ 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)