Re: [PATCH v15 03/13] x86/sev: Add Secure TSC support for SNP guests

Tom Lendacky
2024-12-16T16:06:56Z

On 12/3/24 03:00, 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>
> Tested-by: Peter Gonda <pgonda@google.com>
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

Just some minor nits if you have to respin...

> ---
> arch/x86/include/asm/sev-common.h | 1 +
> arch/x86/include/asm/sev.h | 22 ++++++
> arch/x86/include/asm/svm.h | 6 +-
> include/linux/cc_platform.h | 8 +++
> arch/x86/coco/core.c | 3 +
> arch/x86/coco/sev/core.c | 116 ++++++++++++++++++++++++++++++
> arch/x86/mm/mem_encrypt.c | 2 +
> 7 files changed, 156 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 53f3048f484e..9fd02efef08e 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 +177,22 @@ struct snp_guest_msg {
> u8 payload[PAGE_SIZE - sizeof(struct snp_guest_msg_hdr)];
> } __packed;
>
> +#define SNP_TSC_INFO_REQ_SZ 128
> +#define SNP_TSC_INFO_RESP_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;
> @@ -473,6 +492,8 @@ 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
> @@ -514,6 +535,7 @@ 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..cb7103dc124f 100644
> --- a/include/linux/cc_platform.h
> +++ b/include/linux/cc_platform.h
> @@ -88,6 +88,14 @@ enum cc_attr {
> * enabled to run SEV-SNP guests.
> */
> CC_ATTR_HOST_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,

Maybe move this up above the host related attribute so that it is grouped
with the other guest attributes.

> };
>
> #ifdef CONFIG_ARCH_HAS_CC_PLATFORM
> diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
> index 0f81f70aca82..5b9a358a3254 100644
> --- a/arch/x86/coco/core.c
> +++ b/arch/x86/coco/core.c
> @@ -100,6 +100,9 @@ static bool noinstr amd_cc_platform_has(enum cc_attr attr)
> case CC_ATTR_HOST_SEV_SNP:
> return cc_flags.host_sev_snp;
>
> + case CC_ATTR_GUEST_SNP_SECURE_TSC:
> + return sev_status & MSR_AMD64_SNP_SECURE_TSC;
> +

Ditto here. Move this up above the host check.

Also, should this be:

return (sev_status & MSR_AMD64_SEV_SNP_ENABLED) &&
(sev_status & MSR_AMD64_SNP_SECURE_TSC);

?

> default:
> return false;
> }
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index a61898c7f114..39683101b526 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 BP 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;
> @@ -1277,6 +1285,12 @@ 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) {
> @@ -3127,3 +3141,105 @@ int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req
> }
> 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;
> + unsigned char *buf;
> + int rc = -ENOMEM;
> +
> + tsc_req = kzalloc(sizeof(*tsc_req), GFP_KERNEL);
> + if (!tsc_req)
> + return rc;
> +
> + tsc_resp = kzalloc(sizeof(*tsc_resp), 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;
> +
> + /*
> + * The intermediate response buffer is used while decrypting the
> + * response payload. Make sure that it has enough space to cover
> + * the authtag.
> + */
> + buf = kzalloc(SNP_TSC_INFO_RESP_SZ + AUTHTAG_LEN, GFP_KERNEL);
> + if (!buf)
> + goto e_free_rio;
> +
> + mdesc = snp_msg_alloc();
> + if (IS_ERR_OR_NULL(mdesc))
> + goto e_free_buf;
> +
> + 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 = buf;
> + 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;
> +
> + memcpy(tsc_resp, buf, sizeof(*tsc_resp));
> + 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 == 0) {
> + 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(buf, sizeof(buf));
> + memzero_explicit(tsc_resp, sizeof(*tsc_resp));
> +e_free_mdesc:
> + snp_msg_free(mdesc);
> +e_free_buf:
> + kfree(buf);
> +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_SEV_SNP) ||
> + !cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC))

If you make the change above, you only need to check for SNP_SECURE_TSC.

Thanks,
Tom

> + 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();
> }
>