[3/3] x86/sev: Add SEV-SNP CipherTextHiding support

From: Ashish Kalra <ashish.kalra@amd.com>

Ciphertext hiding prevents host accesses from reading the ciphertext of
SNP guest private memory. Instead of reading ciphertext, the host reads
will see constant default values (0xff).

Ciphertext hiding separates the ASID space into SNP guest ASIDs and host
ASIDs. All SNP active guests must have an ASID less than or equal to
MAX_SNP_ASID provided to the SNP_INIT_EX command. All SEV-legacy guests
(SEV and SEV-ES) must be greater than MAX_SNP_ASID.

This patch-set adds a new module parameter to the CCP driver defined as
psp_max_snp_asid which is a user configurable MAX_SNP_ASID to define the
system-wide maximum SNP ASID value. If this value is not set, then the
ASID space is equally divided between SEV-SNP and SEV-ES guests.

Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
---
 arch/x86/kvm/svm/sev.c       | 24 ++++++++++++++---
 drivers/crypto/ccp/sev-dev.c | 50 ++++++++++++++++++++++++++++++++++++
 include/linux/psp-sev.h      | 10 ++++++--
 3 files changed, 79 insertions(+), 5 deletions(-)

On 8/12/24 14:42, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@amd.com>
> 
> Ciphertext hiding prevents host accesses from reading the ciphertext of
> SNP guest private memory. Instead of reading ciphertext, the host reads
> will see constant default values (0xff).
> 
> Ciphertext hiding separates the ASID space into SNP guest ASIDs and host
> ASIDs. All SNP active guests must have an ASID less than or equal to
> MAX_SNP_ASID provided to the SNP_INIT_EX command. All SEV-legacy guests
> (SEV and SEV-ES) must be greater than MAX_SNP_ASID.
> 
> This patch-set adds a new module parameter to the CCP driver defined as
> psp_max_snp_asid which is a user configurable MAX_SNP_ASID to define the
> system-wide maximum SNP ASID value. If this value is not set, then the
> ASID space is equally divided between SEV-SNP and SEV-ES guests.

Add something here to talk about how cipher text hiding needs to be
enabled on SNP_INIT_EX and so the module parameters are part of the CCP
driver.

Not sure if having KVM invoke the CCP, at KVM module load time, to
perform the SNP_INIT_EX would make sense or not. That would allow all
the cipher text hiding support parameters to be in KVM. Lets see what
others think.

> 
> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> ---
>  arch/x86/kvm/svm/sev.c       | 24 ++++++++++++++---
>  drivers/crypto/ccp/sev-dev.c | 50 ++++++++++++++++++++++++++++++++++++
>  include/linux/psp-sev.h      | 10 ++++++--
>  3 files changed, 79 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 532df12b43c5..954ef99a1aa8 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -173,6 +173,9 @@ static void sev_misc_cg_uncharge(struct kvm_sev_info *sev)
>  
>  static int sev_asid_new(struct kvm_sev_info *sev)
>  {
> +	struct kvm_svm *svm = container_of(sev, struct kvm_svm, sev_info);
> +	struct kvm *kvm = &svm->kvm;

Why not just add the vm-type to the input parameters?

> +
>  	/*
>  	 * SEV-enabled guests must use asid from min_sev_asid to max_sev_asid.
>  	 * SEV-ES-enabled guest can use from 1 to min_sev_asid - 1.
> @@ -199,6 +202,18 @@ static int sev_asid_new(struct kvm_sev_info *sev)
>  
>  	mutex_lock(&sev_bitmap_lock);
>  
> +	/*
> +	 * When CipherTextHiding is enabled, all SNP guests must have an
> +	 * ASID less than or equal to MAX_SNP_ASID provided on the
> +	 * SNP_INIT_EX command and all the SEV-ES guests must have
> +	 * an ASID greater than MAX_SNP_ASID.
> +	 */
> +	if (snp_cipher_text_hiding_en && sev->es_active) {
> +		if (kvm->arch.vm_type == KVM_X86_SNP_VM)
> +			max_asid = max_snp_asid;
> +		else
> +			min_asid = max_snp_asid + 1;
> +	}
>  again:
>  	asid = find_next_zero_bit(sev_asid_bitmap, max_asid + 1, min_asid);
>  	if (asid > max_asid) {
> @@ -3058,14 +3073,17 @@ void __init sev_hardware_setup(void)
>  								       "unusable" :
>  								       "disabled",
>  			min_sev_asid, max_sev_asid);
> -	if (boot_cpu_has(X86_FEATURE_SEV_ES))
> +	if (boot_cpu_has(X86_FEATURE_SEV_ES)) {
> +		if (max_snp_asid >= (min_sev_asid - 1))
> +			sev_es_supported = false;
>  		pr_info("SEV-ES %s (ASIDs %u - %u)\n",
>  			sev_es_supported ? "enabled" : "disabled",
> -			min_sev_asid > 1 ? 1 : 0, min_sev_asid - 1);
> +			min_sev_asid > 1 ? max_snp_asid ? max_snp_asid + 1 : 1 : 0, min_sev_asid - 1);

Maybe put the last parameter on a separate line to make this a little
easier to read?

> +	}
>  	if (boot_cpu_has(X86_FEATURE_SEV_SNP))
>  		pr_info("SEV-SNP %s (ASIDs %u - %u)\n",
>  			sev_snp_supported ? "enabled" : "disabled",
> -			min_sev_asid > 1 ? 1 : 0, min_sev_asid - 1);
> +			min_sev_asid > 1 ? 1 : 0, max_snp_asid ? : min_sev_asid - 1);
>  
>  	sev_enabled = sev_supported;
>  	sev_es_enabled = sev_es_supported;
> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
> index eefb481db5af..9ee81a6defc5 100644
> --- a/drivers/crypto/ccp/sev-dev.c
> +++ b/drivers/crypto/ccp/sev-dev.c
> @@ -73,11 +73,27 @@ static bool psp_init_on_probe = true;
>  module_param(psp_init_on_probe, bool, 0444);
>  MODULE_PARM_DESC(psp_init_on_probe, "  if true, the PSP will be initialized on module init. Else the PSP will be initialized on the first command requiring it");
>  
> +static bool psp_cth_enabled = true;

This is a static, so how about just calling this cipher_text_hiding.

> +module_param(psp_cth_enabled, bool, 0444);
> +MODULE_PARM_DESC(psp_cth_enabled, "  if true, the PSP will enable Cipher Text Hiding");
> +
> +static int psp_max_snp_asid;

Also a static, so just call this max_snp_asid.

> +module_param(psp_max_snp_asid, int, 0444);
> +MODULE_PARM_DESC(psp_max_snp_asid, "  override MAX_SNP_ASID for Cipher Text Hiding");
> +
>  MODULE_FIRMWARE("amd/amd_sev_fam17h_model0xh.sbin"); /* 1st gen EPYC */
>  MODULE_FIRMWARE("amd/amd_sev_fam17h_model3xh.sbin"); /* 2nd gen EPYC */
>  MODULE_FIRMWARE("amd/amd_sev_fam19h_model0xh.sbin"); /* 3rd gen EPYC */
>  MODULE_FIRMWARE("amd/amd_sev_fam19h_model1xh.sbin"); /* 4th gen EPYC */
>  
> +/* Cipher Text Hiding Enabled */
> +bool snp_cipher_text_hiding_en;
> +EXPORT_SYMBOL(snp_cipher_text_hiding_en);

I think you drop the "_en" and just have snp_cipher_text_hiding.

> +
> +/* MAX_SNP_ASID */
> +unsigned int max_snp_asid;
> +EXPORT_SYMBOL(max_snp_asid);

This should have "snp_" to provide better namespace isolation.

> +
>  static bool psp_dead;
>  static int psp_timeout;
>  
> @@ -1053,6 +1069,36 @@ static void snp_set_hsave_pa(void *arg)
>  	wrmsrl(MSR_VM_HSAVE_PA, 0);
>  }
>  
> +static void sev_snp_enable_ciphertext_hiding(struct sev_data_snp_init_ex *data, int *error)
> +{
> +	struct psp_device *psp = psp_master;
> +	struct sev_device *sev;
> +	unsigned int edx;
> +
> +	sev = psp->sev_data;
> +
> +	/*
> +	 * Check if CipherTextHiding feature is supported and enabled
> +	 * in the Platform/BIOS.
> +	 */
> +	if (sev->feat_info.ecx & FEAT_CIPHERTEXTHIDING_SUPPORTED &&
> +	    sev->snp_plat_status.ciphertext_hiding_cap) {

I'm not sure you need both checks. Either the platform status or the
feature info check should be enough. Can you check on that?

> +		/* Retrieve SEV CPUID information */
> +		edx = cpuid_edx(0x8000001f);
> +		/* Do sanity checks on user-defined MAX_SNP_ASID */
> +		if (psp_max_snp_asid > (edx - 1)) {

How about:
		if (psp_max_snp_asid >= edx) {

> +			dev_info(sev->dev, "user-defined MAX_SNP_ASID invalid, limiting to %d\n",

max_snp_asid module parameter is not valid, limiting to %d\n

> +				 edx - 1);
> +			psp_max_snp_asid = edx - 1;
> +		}
> +		max_snp_asid = psp_max_snp_asid ? : (edx - 1) / 2;

Add a blank line here

> +		snp_cipher_text_hiding_en = 1;
> +		data->ciphertext_hiding_en = 1;
> +		data->max_snp_asid = max_snp_asid;

Ditto

Thanks,
Tom

> +		dev_dbg(sev->dev, "SEV-SNP CipherTextHiding feature support enabled\n");
> +	}
> +}
> +
>  static void sev_cache_snp_platform_status_and_discover_features(void)
>  {
>  	struct sev_device *sev = psp_master->sev_data;
> @@ -1181,6 +1227,10 @@ static int __sev_snp_init_locked(int *error)
>  		}
>  
>  		memset(&data, 0, sizeof(data));
> +
> +		if (psp_cth_enabled)
> +			sev_snp_enable_ciphertext_hiding(&data, error);
> +
>  		data.init_rmp = 1;
>  		data.list_paddr_en = 1;
>  		data.list_paddr = __psp_pa(snp_range_list);
> diff --git a/include/linux/psp-sev.h b/include/linux/psp-sev.h
> index d46d73911a76..a26b43c2eab9 100644
> --- a/include/linux/psp-sev.h
> +++ b/include/linux/psp-sev.h
> @@ -27,6 +27,9 @@ enum sev_state {
>  	SEV_STATE_MAX
>  };
>  
> +extern bool snp_cipher_text_hiding_en;
> +extern unsigned int max_snp_asid;
> +
>  /**
>   * SEV platform and guest management commands
>   */
> @@ -746,10 +749,13 @@ struct sev_data_snp_guest_request {
>  struct sev_data_snp_init_ex {
>  	u32 init_rmp:1;
>  	u32 list_paddr_en:1;
> -	u32 rsvd:30;
> +	u32 rapl_dis:1;
> +	u32 ciphertext_hiding_en:1;
> +	u32 rsvd:28;
>  	u32 rsvd1;
>  	u64 list_paddr;
> -	u8  rsvd2[48];
> +	u16 max_snp_asid;
> +	u8  rsvd2[46];
>  } __packed;
>  
>  /**

Hi Tom,

On 8/15/2024 10:58 AM, Tom Lendacky wrote:
>> +
>>  static bool psp_dead;
>>  static int psp_timeout;
>>  
>> @@ -1053,6 +1069,36 @@ static void snp_set_hsave_pa(void *arg)
>>  	wrmsrl(MSR_VM_HSAVE_PA, 0);
>>  }
>>  
>> +static void sev_snp_enable_ciphertext_hiding(struct sev_data_snp_init_ex *data, int *error)
>> +{
>> +	struct psp_device *psp = psp_master;
>> +	struct sev_device *sev;
>> +	unsigned int edx;
>> +
>> +	sev = psp->sev_data;
>> +
>> +	/*
>> +	 * Check if CipherTextHiding feature is supported and enabled
>> +	 * in the Platform/BIOS.
>> +	 */
>> +	if (sev->feat_info.ecx & FEAT_CIPHERTEXTHIDING_SUPPORTED &&
>> +	    sev->snp_plat_status.ciphertext_hiding_cap) {
> I'm not sure you need both checks. Either the platform status or the
> feature info check should be enough. Can you check on that?
>
FEATURE_INFO only indicates if SEV FW is capable of ciphertext hiding and SNP_PLATFORM_STATUS ciphertext_hiding_cap bit indicates if the CTH feature is enabled in BIOS, so both checks are needed.

Thanks, Ashish