Re: [PATCH 1/5] KVM: SVM: Decrypt SEV VMSA in dump_vmcb() if debugging is enabled

[Tom Lendacky <thomas.lendacky@amd.com>]

On 3/20/25 08:26, Tom Lendacky wrote:
> An SEV-ES/SEV-SNP VM save area (VMSA) can be decrypted if the guest
> policy allows debugging. Update the dump_vmcb() routine to output
> some of the SEV VMSA contents if possible. This can be useful for
> debug purposes.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  arch/x86/kvm/svm/sev.c | 98 ++++++++++++++++++++++++++++++++++++++++++
>  arch/x86/kvm/svm/svm.c | 13 ++++++
>  arch/x86/kvm/svm/svm.h | 11 +++++
>  3 files changed, 122 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 661108d65ee7..6e3f5042d9ce 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -563,6 +563,8 @@ static int sev_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  	if (copy_from_user(&params, u64_to_user_ptr(argp->data), sizeof(params)))
>  		return -EFAULT;
>  
> +	sev->policy = params.policy;
> +
>  	memset(&start, 0, sizeof(start));
>  
>  	dh_blob = NULL;
> @@ -2220,6 +2222,8 @@ static int snp_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  	if (params.policy & SNP_POLICY_MASK_SINGLE_SOCKET)
>  		return -EINVAL;
>  
> +	sev->policy = params.policy;
> +
>  	sev->snp_context = snp_context_create(kvm, argp);
>  	if (!sev->snp_context)
>  		return -ENOTTY;
> @@ -4975,3 +4979,97 @@ int sev_private_max_mapping_level(struct kvm *kvm, kvm_pfn_t pfn)
>  
>  	return level;
>  }
> +
> +struct vmcb_save_area *sev_decrypt_vmsa(struct kvm_vcpu *vcpu)
> +{
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +	struct vmcb_save_area *vmsa;
> +	struct kvm_sev_info *sev;
> +	int error = 0;
> +	int ret;
> +
> +	if (!sev_es_guest(vcpu->kvm))
> +		return NULL;
> +
> +	/*
> +	 * If the VMSA has not yet been encrypted, return a pointer to the
> +	 * current un-encrypted VMSA.
> +	 */
> +	if (!vcpu->arch.guest_state_protected)
> +		return (struct vmcb_save_area *)svm->sev_es.vmsa;
> +
> +	sev = to_kvm_sev_info(vcpu->kvm);
> +
> +	/* Check if the SEV policy allows debugging */
> +	if (sev_snp_guest(vcpu->kvm)) {
> +		if (!(sev->policy & SNP_POLICY_DEBUG))
> +			return NULL;
> +	} else {
> +		if (sev->policy & SEV_POLICY_NODBG)
> +			return NULL;
> +	}
> +
> +	if (sev_snp_guest(vcpu->kvm)) {
> +		struct sev_data_snp_dbg dbg = {0};
> +
> +		vmsa = snp_alloc_firmware_page(__GFP_ZERO);
> +		if (!vmsa)
> +			return NULL;
> +
> +		dbg.gctx_paddr = __psp_pa(sev->snp_context);
> +		dbg.src_addr = svm->vmcb->control.vmsa_pa;
> +		dbg.dst_addr = __psp_pa(vmsa);
> +
> +		ret = sev_issue_cmd(vcpu->kvm, SEV_CMD_SNP_DBG_DECRYPT, &dbg, &error);
> +
> +		/*
> +		 * Return the target page to a hypervisor page no matter what.
> +		 * If this fails, the page can't be used, so leak it and don't
> +		 * try to use it.
> +		 */
> +		if (snp_page_reclaim(vcpu->kvm, PHYS_PFN(__pa(vmsa))))
> +			return NULL;

And actually I should call snp_leak_pages() here to record that. I'll add
that to the next version.

Thanks,
Tom

> +
> +		if (ret) {
> +			pr_err("SEV: SNP_DBG_DECRYPT failed ret=%d, fw_error=%d (%#x)\n",
> +			       ret, error, error);
> +			free_page((unsigned long)vmsa);
> +
> +			return NULL;
> +		}
> +	} else {
> +		struct sev_data_dbg dbg = {0};
> +		struct page *vmsa_page;
> +
> +		vmsa_page = alloc_page(GFP_KERNEL);
> +		if (!vmsa_page)
> +			return NULL;
> +
> +		vmsa = page_address(vmsa_page);
> +
> +		dbg.handle = sev->handle;
> +		dbg.src_addr = svm->vmcb->control.vmsa_pa;
> +		dbg.dst_addr = __psp_pa(vmsa);
> +		dbg.len = PAGE_SIZE;
> +
> +		ret = sev_issue_cmd(vcpu->kvm, SEV_CMD_DBG_DECRYPT, &dbg, &error);
> +		if (ret) {
> +			pr_err("SEV: SEV_CMD_DBG_DECRYPT failed ret=%d, fw_error=%d (0x%x)\n",
> +			       ret, error, error);
> +			__free_page(vmsa_page);
> +
> +			return NULL;
> +		}
> +	}
> +
> +	return vmsa;
> +}
> +
> +void sev_free_decrypted_vmsa(struct kvm_vcpu *vcpu, struct vmcb_save_area *vmsa)
> +{
> +	/* If the VMSA has not yet been encrypted, nothing was allocated */
> +	if (!vcpu->arch.guest_state_protected || !vmsa)
> +		return;
> +
> +	free_page((unsigned long)vmsa);
> +}
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index e67de787fc71..21477871073c 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3423,6 +3423,15 @@ static void dump_vmcb(struct kvm_vcpu *vcpu)
>  	pr_err("%-20s%016llx\n", "avic_logical_id:", control->avic_logical_id);
>  	pr_err("%-20s%016llx\n", "avic_physical_id:", control->avic_physical_id);
>  	pr_err("%-20s%016llx\n", "vmsa_pa:", control->vmsa_pa);
> +
> +	if (sev_es_guest(vcpu->kvm)) {
> +		save = sev_decrypt_vmsa(vcpu);
> +		if (!save)
> +			goto no_vmsa;
> +
> +		save01 = save;
> +	}
> +
>  	pr_err("VMCB State Save Area:\n");
>  	pr_err("%-5s s: %04x a: %04x l: %08x b: %016llx\n",
>  	       "es:",
> @@ -3493,6 +3502,10 @@ static void dump_vmcb(struct kvm_vcpu *vcpu)
>  	pr_err("%-15s %016llx %-13s %016llx\n",
>  	       "excp_from:", save->last_excp_from,
>  	       "excp_to:", save->last_excp_to);
> +
> +no_vmsa:
> +	if (sev_es_guest(vcpu->kvm))
> +		sev_free_decrypted_vmsa(vcpu, save);
>  }
>  
>  static bool svm_check_exit_valid(u64 exit_code)
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index ea44c1da5a7c..66979ddc3659 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -98,6 +98,7 @@ struct kvm_sev_info {
>  	unsigned int asid;	/* ASID used for this guest */
>  	unsigned int handle;	/* SEV firmware handle */
>  	int fd;			/* SEV device fd */
> +	unsigned long policy;
>  	unsigned long pages_locked; /* Number of pages locked */
>  	struct list_head regions_list;  /* List of registered regions */
>  	u64 ap_jump_table;	/* SEV-ES AP Jump Table address */
> @@ -114,6 +115,9 @@ struct kvm_sev_info {
>  	struct mutex guest_req_mutex; /* Must acquire before using bounce buffers */
>  };
>  
> +#define SEV_POLICY_NODBG	BIT_ULL(0)
> +#define SNP_POLICY_DEBUG	BIT_ULL(19)
> +
>  struct kvm_svm {
>  	struct kvm kvm;
>  
> @@ -756,6 +760,8 @@ void sev_snp_init_protected_guest_state(struct kvm_vcpu *vcpu);
>  int sev_gmem_prepare(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, int max_order);
>  void sev_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end);
>  int sev_private_max_mapping_level(struct kvm *kvm, kvm_pfn_t pfn);
> +struct vmcb_save_area *sev_decrypt_vmsa(struct kvm_vcpu *vcpu);
> +void sev_free_decrypted_vmsa(struct kvm_vcpu *vcpu, struct vmcb_save_area *vmsa);
>  #else
>  static inline struct page *snp_safe_alloc_page_node(int node, gfp_t gfp)
>  {
> @@ -787,6 +793,11 @@ static inline int sev_private_max_mapping_level(struct kvm *kvm, kvm_pfn_t pfn)
>  	return 0;
>  }
>  
> +static inline struct vmcb_save_area *sev_decrypt_vmsa(struct kvm_vcpu *vcpu)
> +{
> +	return NULL;
> +}
> +static inline void sev_free_decrypted_vmsa(struct kvm_vcpu *vcpu, struct vmcb_save_area *vmsa) {}
>  #endif
>  
>  /* vmenter.S */