Re: [PATCH v2 2/2] x86/sgx: Implement EUPDATESVN and opportunistically call it during first EPC page alloc

[Jarkko Sakkinen]

On Fri, Mar 28, 2025 at 02:57:41PM +0200, Elena Reshetova wrote:
> SGX architecture introduced a new instruction called EUPDATESVN
> to Ice Lake. It allows updating security SVN version, given that EPC
> is completely empty. The latter is required for security reasons
> in order to reason that enclave security posture is as secure as the
> security SVN version of the TCB that created it.
> 
> Additionally it is important to ensure that while ENCLS[EUPDATESVN]
> runs, no concurrent page creation happens in EPC, because it might
> result in #GP delivered to the creator. Legacy SW might not be prepared
> to handle such unexpected #GPs and therefore this patch introduces
> a locking mechanism to ensure no concurrent EPC allocations can happen.
> 
> It is also ensured that ENCLS[EUPDATESVN] is not called when running
> in a VM since it does not have a meaning in this context (microcode
> updates application is limited to the host OS) and will create
> unnecessary load.
> 
> This patch is based on previous submision by Cathy Zhang
> https://lore.kernel.org/all/20220520103904.1216-1-cathy.zhang@xxxxxxxxx/
> 
> Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> ---
>  arch/x86/include/asm/sgx.h      | 41 +++++++++++++--------
>  arch/x86/kernel/cpu/sgx/encls.h |  6 ++++
>  arch/x86/kernel/cpu/sgx/main.c  | 63 ++++++++++++++++++++++++++++++++-
>  arch/x86/kernel/cpu/sgx/sgx.h   |  1 +
>  4 files changed, 95 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/include/asm/sgx.h b/arch/x86/include/asm/sgx.h
> index 6a0069761508..5caf5c31ebc6 100644
> --- a/arch/x86/include/asm/sgx.h
> +++ b/arch/x86/include/asm/sgx.h
> @@ -26,23 +26,26 @@
>  #define SGX_CPUID_EPC_SECTION	0x1
>  /* The bitmask for the EPC section type. */
>  #define SGX_CPUID_EPC_MASK	GENMASK(3, 0)
> +/* EUPDATESVN presence indication */
> +#define SGX_CPUID_EUPDATESVN	BIT(10)
>  
>  enum sgx_encls_function {
> -	ECREATE	= 0x00,
> -	EADD	= 0x01,
> -	EINIT	= 0x02,
> -	EREMOVE	= 0x03,
> -	EDGBRD	= 0x04,
> -	EDGBWR	= 0x05,
> -	EEXTEND	= 0x06,
> -	ELDU	= 0x08,
> -	EBLOCK	= 0x09,
> -	EPA	= 0x0A,
> -	EWB	= 0x0B,
> -	ETRACK	= 0x0C,
> -	EAUG	= 0x0D,
> -	EMODPR	= 0x0E,
> -	EMODT	= 0x0F,
> +	ECREATE		= 0x00,
> +	EADD		= 0x01,
> +	EINIT		= 0x02,
> +	EREMOVE		= 0x03,
> +	EDGBRD		= 0x04,
> +	EDGBWR		= 0x05,
> +	EEXTEND		= 0x06,
> +	ELDU		= 0x08,
> +	EBLOCK		= 0x09,
> +	EPA		= 0x0A,
> +	EWB		= 0x0B,
> +	ETRACK		= 0x0C,
> +	EAUG		= 0x0D,
> +	EMODPR		= 0x0E,
> +	EMODT		= 0x0F,
> +	EUPDATESVN	= 0x18,
>  };
>  
>  /**
> @@ -73,6 +76,11 @@ enum sgx_encls_function {
>   *				public key does not match IA32_SGXLEPUBKEYHASH.
>   * %SGX_PAGE_NOT_MODIFIABLE:	The EPC page cannot be modified because it
>   *				is in the PENDING or MODIFIED state.
> + * %SGX_INSUFFICIENT_ENTROPY:	Insufficient entropy in RNG.
> + * %SGX_EPC_NOT_READY:		EPC is not ready for SVN update.
> + * %SGX_NO_UPDATE:		EUPDATESVN was successful, but CPUSVN was not
> + *				updated because current SVN was not newer than
> + *				CPUSVN.
>   * %SGX_UNMASKED_EVENT:		An unmasked event, e.g. INTR, was received
>   */
>  enum sgx_return_code {
> @@ -81,6 +89,9 @@ enum sgx_return_code {
>  	SGX_CHILD_PRESENT		= 13,
>  	SGX_INVALID_EINITTOKEN		= 16,
>  	SGX_PAGE_NOT_MODIFIABLE		= 20,
> +	SGX_INSUFFICIENT_ENTROPY	= 29,
> +	SGX_EPC_NOT_READY		= 30,
> +	SGX_NO_UPDATE			= 31,
>  	SGX_UNMASKED_EVENT		= 128,
>  };
>  
> diff --git a/arch/x86/kernel/cpu/sgx/encls.h b/arch/x86/kernel/cpu/sgx/encls.h
> index 99004b02e2ed..3d83c76dc91f 100644
> --- a/arch/x86/kernel/cpu/sgx/encls.h
> +++ b/arch/x86/kernel/cpu/sgx/encls.h
> @@ -233,4 +233,10 @@ static inline int __eaug(struct sgx_pageinfo *pginfo, void *addr)
>  	return __encls_2(EAUG, pginfo, addr);
>  }
>  
> +/* Update CPUSVN at runtime. */
> +static inline int __eupdatesvn(void)
> +{
> +	return __encls_ret_1(EUPDATESVN, "");
> +}
> +
>  #endif /* _X86_ENCLS_H */
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index b61d3bad0446..24563110811d 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -32,6 +32,11 @@ static DEFINE_XARRAY(sgx_epc_address_space);
>  static LIST_HEAD(sgx_active_page_list);
>  static DEFINE_SPINLOCK(sgx_reclaimer_lock);
>  
> +/* This lock is held to prevent new EPC pages from being created
> + * during the execution of ENCLS[EUPDATESVN].
> + */
> +static DEFINE_SPINLOCK(sgx_epc_eupdatesvn_lock);
> +
>  static atomic_long_t sgx_nr_used_pages = ATOMIC_LONG_INIT(0);
>  static unsigned long sgx_nr_total_pages;
>  
> @@ -457,7 +462,17 @@ static struct sgx_epc_page *__sgx_alloc_epc_page_from_node(int nid)
>  	page->flags = 0;
>  
>  	spin_unlock(&node->lock);
> -	atomic_long_inc(&sgx_nr_used_pages);
> +
> +	if (!atomic_long_inc_not_zero(&sgx_nr_used_pages)) {
> +		spin_lock(&sgx_epc_eupdatesvn_lock);
> +		/* Only call sgx_updatesvn() once the first enclave's
> +		 * page is allocated from EPC
> +		 */
> +		if (atomic_long_read(&sgx_nr_used_pages) == 0)
> +			sgx_updatesvn();
> +		atomic_long_inc(&sgx_nr_used_pages);
> +		spin_unlock(&sgx_epc_eupdatesvn_lock);
> +	}
>  
>  	return page;
>  }
> @@ -970,3 +985,49 @@ static int __init sgx_init(void)
>  }
>  
>  device_initcall(sgx_init);
> +
> +/**
> + * sgx_updatesvn() - Issue ENCLS[EUPDATESVN]
> + * If EPC is ready, this instruction will update CPUSVN to the currently
> + * loaded microcode update SVN and generate new cryptographic assets.
> + */
> +void sgx_updatesvn(void)
> +{
> +	int retry = 10;
> +	int ret;
> +
> +	lockdep_assert_held(&sgx_epc_eupdatesvn_lock);
> +
> +	if (!(cpuid_eax(SGX_CPUID) & SGX_CPUID_EUPDATESVN))
> +		return;
> +
> +	/* Do not execute ENCLS[EUPDATESVN] if running in a VM since
> +	 * microcode updates are only meaningful to be applied on the host.
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
> +		return;
> +
> +	do {
> +		ret = __eupdatesvn();
> +		if (ret != SGX_INSUFFICIENT_ENTROPY)
> +			break;
> +
> +	} while (--retry);
> +
> +	switch (ret) {
> +	case 0:
> +		pr_info("EUPDATESVN: success\n");
> +		break;
> +	case SGX_EPC_NOT_READY:
> +	case SGX_INSUFFICIENT_ENTROPY:
> +	case SGX_EPC_PAGE_CONFLICT:
> +		pr_err("EUPDATESVN: error %d\n", ret);
> +		break;
> +	case SGX_NO_UPDATE:
> +		break;
> +	default:
> +		pr_err("EUPDATESVN: unknown error %d\n", ret);
> +		break;
> +	}

Overall, I think you're right in that "inversion" does make sense,
now that other stuff is better aligned.

At least when there is spurious error, I think ioctl's should stop
responding and driver should not do anything useful anymore. I.e.,
it should go out-of-service.

I don't think the driver should tear-down, just stop servicing
VM's and responding ioctl's.

Possibly thish should be also right action for other errors than
"insufficient entropy" but I'm open for comments for this.

BR, Jarkko