RE: [PATCH v11 4/5] x86/sgx: Implement ENCLS[EUPDATESVN]

[Reshetova, Elena]

> -----Original Message-----
> From: Huang, Kai <kai.huang@xxxxxxxxx>
> Sent: Thursday, August 7, 2025 3:15 AM
> To: Reshetova, Elena <elena.reshetova@xxxxxxxxx>; Hansen, Dave
> <dave.hansen@xxxxxxxxx>
> Cc: seanjc@xxxxxxxxxx; mingo@xxxxxxxxxx; Scarlata, Vincent R
> <vincent.r.scarlata@xxxxxxxxx>; x86@xxxxxxxxxx; jarkko@xxxxxxxxxx;
> Annapurve, Vishal <vannapurve@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> Mallick, Asit K <asit.k.mallick@xxxxxxxxx>; Aktas, Erdem
> <erdemaktas@xxxxxxxxxx>; Cai, Chong <chongc@xxxxxxxxxx>; Bondarevska,
> Nataliia <bondarn@xxxxxxxxxx>; linux-sgx@xxxxxxxxxxxxxxx; Raynor, Scott
> <scott.raynor@xxxxxxxxx>
> Subject: Re: [PATCH v11 4/5] x86/sgx: Implement ENCLS[EUPDATESVN]
> 
> On Wed, 2025-08-06 at 11:11 +0300, Elena Reshetova wrote:
> > All running enclaves and cryptographic assets (such as internal SGX
> > encryption keys) are assumed to be compromised whenever an SGX-related
> > microcode update occurs. To mitigate this assumed compromise the new
> > supervisor SGX instruction ENCLS[EUPDATESVN] can generate fresh
> > cryptographic assets.
> >
> > Before executing EUPDATESVN, all SGX memory must be marked as unused.
> > This requirement ensures that no potentially compromised enclave
> > survives the update and allows the system to safely regenerate
> > cryptographic assets.
> >
> > Add the method to perform ENCLS[EUPDATESVN]. However, until the
> > follow up patch that wires calling sgx_update_svn() from
> > sgx_inc_usage_count(), this code is not reachable.
> 
> Please check the text wrap.
> 

Yes, will do. 

> >
> > Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> > ---
> >  arch/x86/include/asm/sgx.h      | 31 +++++++-------
> >  arch/x86/kernel/cpu/sgx/encls.h |  5 +++
> >  arch/x86/kernel/cpu/sgx/main.c  | 73
> +++++++++++++++++++++++++++++++++
> >  3 files changed, 94 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/sgx.h b/arch/x86/include/asm/sgx.h
> > index 2da5b3b117a1..0e13678f9cbd 100644
> > --- a/arch/x86/include/asm/sgx.h
> > +++ b/arch/x86/include/asm/sgx.h
> > @@ -28,21 +28,22 @@
> >  #define SGX_CPUID_EPC_MASK	GENMASK(3, 0)
> >
> >  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,
> >  };
> >
> >  /**
> > diff --git a/arch/x86/kernel/cpu/sgx/encls.h
> b/arch/x86/kernel/cpu/sgx/encls.h
> > index 99004b02e2ed..d9160c89a93d 100644
> > --- a/arch/x86/kernel/cpu/sgx/encls.h
> > +++ b/arch/x86/kernel/cpu/sgx/encls.h
> > @@ -233,4 +233,9 @@ static inline int __eaug(struct sgx_pageinfo *pginfo,
> void *addr)
> >  	return __encls_2(EAUG, pginfo, addr);
> >  }
> >
> > +/* Attempt to 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 3a5cbd1c170e..d8c42524b590 100644
> > --- a/arch/x86/kernel/cpu/sgx/main.c
> > +++ b/arch/x86/kernel/cpu/sgx/main.c
> > @@ -16,6 +16,7 @@
> >  #include <linux/vmalloc.h>
> >  #include <asm/msr.h>
> >  #include <asm/sgx.h>
> > +#include <asm/archrandom.h>
> >  #include "driver.h"
> >  #include "encl.h"
> >  #include "encls.h"
> > @@ -917,6 +918,78 @@ int sgx_set_attribute(unsigned long
> *allowed_attributes,
> >  }
> >  EXPORT_SYMBOL_GPL(sgx_set_attribute);
> >
> > +/* Counter to count the active SGX users */
> > +static int sgx_usage_count;
> > +
> > +/**
> > + * sgx_update_svn() - Attempt to call ENCLS[EUPDATESVN].
> 
> Add a newline would make the comment more readable.

Ok

> 
> > + * This instruction attempts to update CPUSVN to the
> > + * currently loaded microcode update SVN and generate new
> > + * cryptographic assets.
> 
> Ditto here.

Ok

> 
> > + * Return:
> > + * 0: Success or not supported
> > + * -EAGAIN: Can be safely retried, failure is due to lack of
> > + *  entropy in RNG.
> > + * -EIO: Unexpected error, retries are not advisable.
> > + */
> 
> You may want to make the description of the return code vertically
> aligned.  And looking at the k-doc documentation, the format of the return
> codes could be improved:
> 
> 	* Return:
> 	* * %0:		- Success or not supported
> 	* * %-EAGAIN:	- ...
> 	* * %-EIO:	- ...
> 
> Please see "Return values" part of:
> 
> https://docs.kernel.org/doc-guide/kernel-doc.html
> 
> Or you can switch to use normal comment.  It's a static function anyway.

Ok, will fix the formatting issues. 

> 
> 
> > +static int __maybe_unused sgx_update_svn(void)
> > +{
> > +	int ret;
> > +
> > +	/*
> > +	 * If EUPDATESVN is not available, it is ok to
> > +	 * silently skip it to comply with legacy behavior.
> > +	 */
> > +	if (!cpu_feature_enabled(X86_FEATURE_SGX_EUPDATESVN))
> > +		return 0;
> > +
> > +	/*
> > +	 * EPC is guaranteed to be empty when there are no users.
> > +	 * Ensure we are on our first user before proceeding further.
> > +	 */
> > +	WARN(sgx_usage_count != 1, "Elevated usage count when calling
> EUPDATESVN\n");
> 
> I am not sure whether this is needed.  Wouldn't the ENCLS_WARN() at the
> end catch this case and the user is able to figure out what went wrong
> from the error code?

Dave has made a suggestion to include this check, so I have added it. 

> 
> Besides that, in _this_ patch, what prevents sgx_usage_count from being
> concurrently updated is still unknown.  It's kinda weird to just use it
> here w/o seeing the actual mutex.

In this patch it is fully useless, because sgx_usage_count is never incremented
from zero and this function is also never called. But I didn’t want to move this
addition to the following patch since it would look as one-add to this function. 

> 
> > +
> > +	for (int i = 0; i < RDRAND_RETRY_LOOPS; i++) {
> > +		ret = __eupdatesvn();
> > +
> > +		/* Stop on success or unexpected errors: */
> > +		if (ret != SGX_INSUFFICIENT_ENTROPY)
> > +			break;
> > +	}
> > +
> > +	switch (ret) {
> > +	/*
> > +	 * SVN successfully updated.
> > +	 * Let users know when the update was successful.
> > +	 */
> 
> This is ugly.  I would just put the comment inside the 'case'.

Ok, will move. 

> 
> > +	case 0:
> > +		pr_info("SVN updated successfully\n");
> > +		return 0;
> > +	/*
> > +	 * SVN update failed since the current SVN is
> > +	 * not newer than CPUSVN. This is the most
> > +	 * common case and indicates no harm.
> > +	 */
> > +	case SGX_NO_UPDATE:
> > +		return 0;
> > +	/*
> > +	 * SVN update failed due to lack of entropy in DRNG.
> > +	 * Indicate to userspace that it should retry.
> > +	 */
> > +	case SGX_INSUFFICIENT_ENTROPY:
> > +		return -EAGAIN;
> > +	default:
> > +		break;
> > +	}
> 
> Ditto for all the comments above.

Ok, will move inside. 

Best Regards,
Elena.