Re: [PATCH v16 05/13] x86/sev: Add Secure TSC support for SNP guests

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

On 1/6/25 06:46, Nikunj A Dadhania wrote:
> Add support for Secure TSC in SNP-enabled guests. Secure TSC allows guests
> to securely use RDTSC/RDTSCP instructions, ensuring that the parameters
> used cannot be altered by the hypervisor once the guest is launched.
> 
> Secure TSC-enabled guests need to query TSC information from the AMD
> Security Processor. This communication channel is encrypted between the AMD
> Security Processor and the guest, with the hypervisor acting merely as a
> conduit to deliver the guest messages to the AMD Security Processor. Each
> message is protected with AEAD (AES-256 GCM).
> 
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

Although if you have to re-spin, it might make sense to use the __free()
support to reduce the number of gotos and labels in snp_get_tsc_info().
You could also use kfree_sensitive() to automatically do the
memzero_explicit(), too.

> ---
>  arch/x86/include/asm/sev-common.h |   1 +
>  arch/x86/include/asm/sev.h        |  21 ++++++
>  arch/x86/include/asm/svm.h        |   6 +-
>  include/linux/cc_platform.h       |   8 +++
>  arch/x86/coco/core.c              |   4 ++
>  arch/x86/coco/sev/core.c          | 107 ++++++++++++++++++++++++++++++
>  arch/x86/mm/mem_encrypt.c         |   2 +
>  7 files changed, 147 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
> index 50f5666938c0..6ef92432a5ce 100644
> --- a/arch/x86/include/asm/sev-common.h
> +++ b/arch/x86/include/asm/sev-common.h
> @@ -206,6 +206,7 @@ struct snp_psc_desc {
>  #define GHCB_TERM_NO_SVSM		7	/* SVSM is not advertised in the secrets page */
>  #define GHCB_TERM_SVSM_VMPL0		8	/* SVSM is present but has set VMPL to 0 */
>  #define GHCB_TERM_SVSM_CAA		9	/* SVSM is present but CAA is not page aligned */
> +#define GHCB_TERM_SECURE_TSC		10	/* Secure TSC initialization failed */
>  
>  #define GHCB_RESP_CODE(v)		((v) & GHCB_MSR_INFO_MASK)
>  
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index 0937ac7a96db..bdcdaac4df1c 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -146,6 +146,9 @@ enum msg_type {
>  	SNP_MSG_VMRK_REQ,
>  	SNP_MSG_VMRK_RSP,
>  
> +	SNP_MSG_TSC_INFO_REQ = 17,
> +	SNP_MSG_TSC_INFO_RSP,
> +
>  	SNP_MSG_TYPE_MAX
>  };
>  
> @@ -174,6 +177,21 @@ struct snp_guest_msg {
>  	u8 payload[PAGE_SIZE - sizeof(struct snp_guest_msg_hdr)];
>  } __packed;
>  
> +#define SNP_TSC_INFO_REQ_SZ	128
> +
> +struct snp_tsc_info_req {
> +	u8 rsvd[SNP_TSC_INFO_REQ_SZ];
> +} __packed;
> +
> +struct snp_tsc_info_resp {
> +	u32 status;
> +	u32 rsvd1;
> +	u64 tsc_scale;
> +	u64 tsc_offset;
> +	u32 tsc_factor;
> +	u8 rsvd2[100];
> +} __packed;
> +
>  struct snp_guest_req {
>  	void *req_buf;
>  	size_t req_sz;
> @@ -463,6 +481,8 @@ void snp_msg_free(struct snp_msg_desc *mdesc);
>  int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req,
>  			   struct snp_guest_request_ioctl *rio);
>  
> +void __init snp_secure_tsc_prepare(void);
> +
>  #else	/* !CONFIG_AMD_MEM_ENCRYPT */
>  
>  #define snp_vmpl 0
> @@ -503,6 +523,7 @@ static inline struct snp_msg_desc *snp_msg_alloc(void) { return NULL; }
>  static inline void snp_msg_free(struct snp_msg_desc *mdesc) { }
>  static inline int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req,
>  					 struct snp_guest_request_ioctl *rio) { return -ENODEV; }
> +static inline void __init snp_secure_tsc_prepare(void) { }
>  
>  #endif	/* CONFIG_AMD_MEM_ENCRYPT */
>  
> diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
> index 2b59b9951c90..92e18798f197 100644
> --- a/arch/x86/include/asm/svm.h
> +++ b/arch/x86/include/asm/svm.h
> @@ -417,7 +417,9 @@ struct sev_es_save_area {
>  	u8 reserved_0x298[80];
>  	u32 pkru;
>  	u32 tsc_aux;
> -	u8 reserved_0x2f0[24];
> +	u64 tsc_scale;
> +	u64 tsc_offset;
> +	u8 reserved_0x300[8];
>  	u64 rcx;
>  	u64 rdx;
>  	u64 rbx;
> @@ -564,7 +566,7 @@ static inline void __unused_size_checks(void)
>  	BUILD_BUG_RESERVED_OFFSET(sev_es_save_area, 0x1c0);
>  	BUILD_BUG_RESERVED_OFFSET(sev_es_save_area, 0x248);
>  	BUILD_BUG_RESERVED_OFFSET(sev_es_save_area, 0x298);
> -	BUILD_BUG_RESERVED_OFFSET(sev_es_save_area, 0x2f0);
> +	BUILD_BUG_RESERVED_OFFSET(sev_es_save_area, 0x300);
>  	BUILD_BUG_RESERVED_OFFSET(sev_es_save_area, 0x320);
>  	BUILD_BUG_RESERVED_OFFSET(sev_es_save_area, 0x380);
>  	BUILD_BUG_RESERVED_OFFSET(sev_es_save_area, 0x3f0);
> diff --git a/include/linux/cc_platform.h b/include/linux/cc_platform.h
> index caa4b4430634..0bf7d33a1048 100644
> --- a/include/linux/cc_platform.h
> +++ b/include/linux/cc_platform.h
> @@ -81,6 +81,14 @@ enum cc_attr {
>  	 */
>  	CC_ATTR_GUEST_SEV_SNP,
>  
> +	/**
> +	 * @CC_ATTR_GUEST_SNP_SECURE_TSC: SNP Secure TSC is active.
> +	 *
> +	 * The platform/OS is running as a guest/virtual machine and actively
> +	 * using AMD SEV-SNP Secure TSC feature.
> +	 */
> +	CC_ATTR_GUEST_SNP_SECURE_TSC,
> +
>  	/**
>  	 * @CC_ATTR_HOST_SEV_SNP: AMD SNP enabled on the host.
>  	 *
> diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
> index 0f81f70aca82..715c2c09582f 100644
> --- a/arch/x86/coco/core.c
> +++ b/arch/x86/coco/core.c
> @@ -97,6 +97,10 @@ static bool noinstr amd_cc_platform_has(enum cc_attr attr)
>  	case CC_ATTR_GUEST_SEV_SNP:
>  		return sev_status & MSR_AMD64_SEV_SNP_ENABLED;
>  
> +	case CC_ATTR_GUEST_SNP_SECURE_TSC:
> +		return (sev_status & MSR_AMD64_SEV_SNP_ENABLED) &&
> +			(sev_status & MSR_AMD64_SNP_SECURE_TSC);
> +
>  	case CC_ATTR_HOST_SEV_SNP:
>  		return cc_flags.host_sev_snp;
>  
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index feb145df6bf7..00a0ac3baab7 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -96,6 +96,14 @@ static u64 sev_hv_features __ro_after_init;
>  /* Secrets page physical address from the CC blob */
>  static u64 secrets_pa __ro_after_init;
>  
> +/*
> + * For Secure TSC guests, the BSP fetches TSC_INFO using SNP guest messaging and
> + * initializes snp_tsc_scale and snp_tsc_offset. These values are replicated
> + * across the APs VMSA fields (TSC_SCALE and TSC_OFFSET).
> + */
> +static u64 snp_tsc_scale __ro_after_init;
> +static u64 snp_tsc_offset __ro_after_init;
> +
>  /* #VC handler runtime per-CPU data */
>  struct sev_es_runtime_data {
>  	struct ghcb ghcb_page;
> @@ -1272,6 +1280,12 @@ static int wakeup_cpu_via_vmgexit(u32 apic_id, unsigned long start_ip)
>  	vmsa->vmpl		= snp_vmpl;
>  	vmsa->sev_features	= sev_status >> 2;
>  
> +	/* Populate AP's TSC scale/offset to get accurate TSC values. */
> +	if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC)) {
> +		vmsa->tsc_scale = snp_tsc_scale;
> +		vmsa->tsc_offset = snp_tsc_offset;
> +	}
> +
>  	/* Switch the page over to a VMSA page now that it is initialized */
>  	ret = snp_set_vmsa(vmsa, caa, apic_id, true);
>  	if (ret) {
> @@ -3121,3 +3135,96 @@ int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(snp_send_guest_request);
> +
> +static int __init snp_get_tsc_info(void)
> +{
> +	struct snp_guest_request_ioctl *rio;
> +	struct snp_tsc_info_resp *tsc_resp;
> +	struct snp_tsc_info_req *tsc_req;
> +	struct snp_msg_desc *mdesc;
> +	struct snp_guest_req *req;
> +	int rc = -ENOMEM;
> +
> +	tsc_req = kzalloc(sizeof(*tsc_req), GFP_KERNEL);
> +	if (!tsc_req)
> +		return rc;
> +
> +	/*
> +	 * The intermediate response buffer is used while decrypting the
> +	 * response payload. Make sure that it has enough space to cover
> +	 * the authtag.
> +	 */
> +	tsc_resp = kzalloc(sizeof(*tsc_resp) + AUTHTAG_LEN, GFP_KERNEL);
> +	if (!tsc_resp)
> +		goto e_free_tsc_req;
> +
> +	req = kzalloc(sizeof(*req), GFP_KERNEL);
> +	if (!req)
> +		goto e_free_tsc_resp;
> +
> +	rio = kzalloc(sizeof(*rio), GFP_KERNEL);
> +	if (!rio)
> +		goto e_free_req;
> +
> +	mdesc = snp_msg_alloc();
> +	if (IS_ERR_OR_NULL(mdesc))
> +		goto e_free_rio;
> +
> +	rc = snp_msg_init(mdesc, snp_vmpl);
> +	if (rc)
> +		goto e_free_mdesc;
> +
> +	req->msg_version = MSG_HDR_VER;
> +	req->msg_type = SNP_MSG_TSC_INFO_REQ;
> +	req->vmpck_id = snp_vmpl;
> +	req->req_buf = tsc_req;
> +	req->req_sz = sizeof(*tsc_req);
> +	req->resp_buf = (void *)tsc_resp;
> +	req->resp_sz = sizeof(*tsc_resp) + AUTHTAG_LEN;
> +	req->exit_code = SVM_VMGEXIT_GUEST_REQUEST;
> +
> +	rc = snp_send_guest_request(mdesc, req, rio);
> +	if (rc)
> +		goto e_request;
> +
> +	pr_debug("%s: response status 0x%x scale 0x%llx offset 0x%llx factor 0x%x\n",
> +		 __func__, tsc_resp->status, tsc_resp->tsc_scale, tsc_resp->tsc_offset,
> +		 tsc_resp->tsc_factor);
> +
> +	if (!tsc_resp->status) {
> +		snp_tsc_scale = tsc_resp->tsc_scale;
> +		snp_tsc_offset = tsc_resp->tsc_offset;
> +	} else {
> +		pr_err("Failed to get TSC info, response status 0x%x\n", tsc_resp->status);
> +		rc = -EIO;
> +	}
> +
> +e_request:
> +	/* The response buffer contains sensitive data, explicitly clear it. */
> +	memzero_explicit(tsc_resp, sizeof(*tsc_resp) + AUTHTAG_LEN);
> +e_free_mdesc:
> +	snp_msg_free(mdesc);
> +e_free_rio:
> +	kfree(rio);
> +e_free_req:
> +	kfree(req);
> + e_free_tsc_resp:
> +	kfree(tsc_resp);
> +e_free_tsc_req:
> +	kfree(tsc_req);
> +
> +	return rc;
> +}
> +
> +void __init snp_secure_tsc_prepare(void)
> +{
> +	if (!cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC))
> +		return;
> +
> +	if (snp_get_tsc_info()) {
> +		pr_alert("Unable to retrieve Secure TSC info from ASP\n");
> +		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_SECURE_TSC);
> +	}
> +
> +	pr_debug("SecureTSC enabled");
> +}
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index 0a120d85d7bb..95bae74fdab2 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -94,6 +94,8 @@ void __init mem_encrypt_init(void)
>  	/* Call into SWIOTLB to update the SWIOTLB DMA buffers */
>  	swiotlb_update_mem_attributes();
>  
> +	snp_secure_tsc_prepare();
> +
>  	print_mem_encrypt_feature_info();
>  }
>