linux-kernel - Re: [PATCH 1/5] KVM: SVM: Decrypt SEV VMSA in dump

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Message-ID: <673b9be2-67b0-7386-0f9a-abfe103e6bc4@amd.com>
Date: Fri, 21 Mar 2025 09:40:11 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org
Cc: Paolo Bonzini <pbonzini@...hat.com>,
 Sean Christopherson <seanjc@...gle.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>,
 Thomas Gleixner <tglx@...utronix.de>, Michael Roth <michael.roth@....com>
Subject: Re: [PATCH 1/5] KVM: SVM: Decrypt SEV VMSA in dump_vmcb() if
 debugging is enabled

On 3/21/25 09:36, Tom Lendacky wrote:
> 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@....com>
>> ---

>> +		/*
>> +		 * 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.

Err... snp_page_reclaim() already does that. Nevermind.

Thanks,
Tom

> 
> 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 */