Re: [PATCH v8 10/16] x86/sev: Add Secure TSC support for SNP guests

["Nikunj A. Dadhania" <nikunj@amd.com>]

On 4/22/2024 7:20 PM, Borislav Petkov wrote:
> On Thu, Feb 15, 2024 at 05:01:22PM +0530, Nikunj A Dadhania wrote:
>> Add support for Secure TSC in SNP enabled guests. Secure TSC allows
>> guest to securely use RDTSC/RDTSCP instructions as the parameters
>> being used cannot be changed by hypervisor once the guest is launched.
>>
>> During the boot-up of the secondary cpus, SecureTSC enabled guests
> 
> "CPUs"

Sure

>> need to query TSC info from AMD Security Processor. This communication
>> channel is encrypted between the AMD Security Processor and the guest,
>> the hypervisor is just the conduit to deliver the guest messages to
>> the AMD Security Processor. Each message is protected with an
>> AEAD (AES-256 GCM). Use minimal AES GCM library to encrypt/decrypt SNP
>> Guest messages to communicate with the PSP.
>>
>> Use the guest enc_init hook to fetch SNP TSC info from the AMD Security
>> Processor and initialize the snp_tsc_scale and snp_tsc_offset. During
>> secondary CPU initialization set VMSA fields GUEST_TSC_SCALE (offset 2F0h)
>> and GUEST_TSC_OFFSET(offset 2F8h) with snp_tsc_scale and snp_tsc_offset
>> respectively.
>>
>> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
>> Tested-by: Peter Gonda <pgonda@google.com>
>> ---
>>  arch/x86/include/asm/sev-common.h |   1 +
>>  arch/x86/include/asm/sev.h        |  23 +++++++
>>  arch/x86/include/asm/svm.h        |   6 +-
>>  arch/x86/kernel/sev.c             | 107 ++++++++++++++++++++++++++++--
>>  arch/x86/mm/mem_encrypt_amd.c     |   6 ++
>>  5 files changed, 134 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
>> index b463fcbd4b90..6adc8e27feeb 100644
>> --- a/arch/x86/include/asm/sev-common.h
>> +++ b/arch/x86/include/asm/sev-common.h
>> @@ -159,6 +159,7 @@ struct snp_psc_desc {
>>  #define GHCB_TERM_NOT_VMPL0		3	/* SNP guest is not running at VMPL-0 */
>>  #define GHCB_TERM_CPUID			4	/* CPUID-validation failure */
>>  #define GHCB_TERM_CPUID_HV		5	/* CPUID failure during hypervisor fallback */
>> +#define GHCB_TERM_SECURE_TSC		6	/* 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 d950a3ac5694..16bf5afa7731 100644
>> --- a/arch/x86/include/asm/sev.h
>> +++ b/arch/x86/include/asm/sev.h
>> @@ -170,6 +170,8 @@ enum msg_type {
>>  	SNP_MSG_ABSORB_RSP,
>>  	SNP_MSG_VMRK_REQ,
>>  	SNP_MSG_VMRK_RSP,
> 
> <-- Pls leave an empty newline here to denote that there's a hole in the
> define numbers. Alternatively, you can add the missing ones too.

I will add empty line.

>> +	SNP_MSG_TSC_INFO_REQ = 17,
>> +	SNP_MSG_TSC_INFO_RSP,
>>  
>>  	SNP_MSG_TYPE_MAX
>>  };
>> @@ -214,6 +216,23 @@ struct sev_guest_platform_data {
>>  	struct snp_req_data input;
>>  };
>>  
>> +#define SNP_TSC_INFO_REQ_SZ 128
>> +
>> +struct snp_tsc_info_req {
>> +	/* Must be zero filled */
> 
> Instead of adding a comment which people might very likely miss, add
> a check for that array to warn when it is not zeroed.

Sure.

> 
>> +	u8 rsvd[SNP_TSC_INFO_REQ_SZ];
>> +} __packed;
>> +
>> +struct snp_tsc_info_resp {
>> +	/* Status of TSC_INFO message */
> 
> The other struct members don't need a comment?

Sure.

>> +	u32 status;
>> +	u32 rsvd1;
>> +	u64 tsc_scale;
>> +	u64 tsc_offset;
>> +	u32 tsc_factor;
>> +	u8 rsvd2[100];
>> +} __packed;
>> +
>>  struct snp_guest_dev {
>>  	struct device *dev;
>>  	struct miscdevice misc;
>> @@ -233,6 +252,7 @@ struct snp_guest_dev {
>>  		struct snp_report_req report;
>>  		struct snp_derived_key_req derived_key;
>>  		struct snp_ext_report_req ext_report;
>> +		struct snp_tsc_info_req tsc_info;
>>  	} req;
>>  	unsigned int vmpck_id;
>>  };
>> @@ -370,6 +390,8 @@ static inline void *alloc_shared_pages(size_t sz)
>>  
>>  	return page_address(page);
>>  }
>> +
>> +void __init snp_secure_tsc_prepare(void);
>>  #else
>>  static inline void sev_es_ist_enter(struct pt_regs *regs) { }
>>  static inline void sev_es_ist_exit(void) { }
>> @@ -404,6 +426,7 @@ static inline int snp_send_guest_request(struct snp_guest_dev *dev, struct snp_g
>>  					 struct snp_guest_request_ioctl *rio) { return 0; }
>>  static inline void free_shared_pages(void *buf, size_t sz) { }
>>  static inline void *alloc_shared_pages(size_t sz) { return NULL; }
>> +static inline void __init snp_secure_tsc_prepare(void) { }
>>  #endif
>>  
>>  #ifdef CONFIG_KVM_AMD_SEV
>> diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
>> index 87a7b917d30e..3a8294bbd109 100644
>> --- a/arch/x86/include/asm/svm.h
>> +++ b/arch/x86/include/asm/svm.h
>> @@ -410,7 +410,9 @@ 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;
>> @@ -542,7 +544,7 @@ 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/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
>> index a9c1efd6d4e3..20a1e50b7638 100644
>> --- a/arch/x86/kernel/sev.c
>> +++ b/arch/x86/kernel/sev.c
>> @@ -75,6 +75,10 @@ static u64 sev_hv_features __ro_after_init;
>>  /* Secrets page physical address from the CC blob */
>>  static u64 secrets_pa __ro_after_init;
>>  
>> +/* Secure TSC values read using TSC_INFO SNP Guest request */
>> +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;
>> @@ -956,6 +960,83 @@ void snp_guest_cmd_unlock(void)
>>  }
>>  EXPORT_SYMBOL_GPL(snp_guest_cmd_unlock);
>>  
>> +static struct snp_guest_dev tsc_snp_dev __initdata;
>> +
>> +static int __snp_send_guest_request(struct snp_guest_dev *snp_dev, struct snp_guest_req *req,
>> +				    struct snp_guest_request_ioctl *rio);
>> +
> 
> Pls design your code without the need for a forward declaration.

Sure

> 
>> +static int __init snp_get_tsc_info(void)
>> +{
>> +	struct snp_tsc_info_req *tsc_req = &tsc_snp_dev.req.tsc_info;
>> +	static u8 buf[SNP_TSC_INFO_REQ_SZ + AUTHTAG_LEN];
>> +	struct snp_guest_request_ioctl rio;
>> +	struct snp_tsc_info_resp tsc_resp;
>> +	struct snp_guest_req req;
>> +	int rc, resp_len;
>> +
>> +	/*
>> +	 * The intermediate response buffer is used while decrypting the
>> +	 * response payload. Make sure that it has enough space to cover the
>> +	 * authtag.
>> +	 */
>> +	resp_len = sizeof(tsc_resp) + AUTHTAG_LEN;
>> +	if (sizeof(buf) < resp_len)
>> +		return -EINVAL;
> 
> Huh, those both are static buffers. Such checks are done with
> BUILD_BUG_ON.

Ok

> 
>> +	memset(tsc_req, 0, sizeof(*tsc_req));
>> +	memset(&req, 0, sizeof(req));
>> +	memset(&rio, 0, sizeof(rio));
>> +	memset(buf, 0, sizeof(buf));
>> +
>> +	if (!snp_assign_vmpck(&tsc_snp_dev, 0))
>> +		return -EINVAL;
> 
> Do that before the memsetting.
> 

Sure

>> +
>> +	/* Initialize the PSP channel to send snp messages */
>> +	rc = snp_setup_psp_messaging(&tsc_snp_dev);
>> +	if (rc)
>> +		return rc;
>> +
>> +	req.msg_version = MSG_HDR_VER;
>> +	req.msg_type = SNP_MSG_TSC_INFO_REQ;
>> +	req.vmpck_id = tsc_snp_dev.vmpck_id;
>> +	req.req_buf = tsc_req;
>> +	req.req_sz = sizeof(*tsc_req);
>> +	req.resp_buf = buf;
>> +	req.resp_sz = resp_len;
>> +	req.exit_code = SVM_VMGEXIT_GUEST_REQUEST;
>> +
>> +	rc = __snp_send_guest_request(&tsc_snp_dev, &req, &rio);
> 
> The changes to *snp_send_guest_request are unrelated to the secure TSC
> enablement. Pls do them in a pre-patch.

Sure

> 
> Ok, I'm going to stop here and give you a chance to work in all the
> review feedback and send a new revision.

Thank you for detailed review/feedback, will address them and send new revision.

Regards
Nikunj