linux-kernel - RE: [PATCH v5 5/5] x86/sgx: Enable automatic SVN updates for SGX enclaves

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Message-ID: <DM8PR11MB57504A6B41329214262E16E2E79CA@DM8PR11MB5750.namprd11.prod.outlook.com>
Date: Mon, 19 May 2025 11:27:07 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: Ingo Molnar <mingo@...nel.org>
CC: "Hansen, Dave" <dave.hansen@...el.com>, "jarkko@...nel.org"
	<jarkko@...nel.org>, "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>, "linux-sgx@...r.kernel.org"
	<linux-sgx@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>, "Mallick,
 Asit K" <asit.k.mallick@...el.com>, "Scarlata, Vincent R"
	<vincent.r.scarlata@...el.com>, "Cai, Chong" <chongc@...gle.com>, "Aktas,
 Erdem" <erdemaktas@...gle.com>, "Annapurve, Vishal" <vannapurve@...gle.com>,
	"dionnaglaze@...gle.com" <dionnaglaze@...gle.com>, "bondarn@...gle.com"
	<bondarn@...gle.com>, "Raynor, Scott" <scott.raynor@...el.com>
Subject: RE: [PATCH v5 5/5] x86/sgx: Enable automatic SVN updates for SGX
 enclaves

 
> * Elena Reshetova <elena.reshetova@...el.com> wrote:
> 
> > @@ -19,10 +19,15 @@ static int sgx_open(struct inode *inode, struct file
> *file)
> >  	struct sgx_encl *encl;
> >  	int ret;
> >
> > +	ret = sgx_inc_usage_count();
> > +	if (ret)
> > +		return -EBUSY;
> 
> So if sgx_inc_usage_count() returns nonzero, it's in use already and we
> return -EBUSY, right?

I guess my selection of error code here was wrong. 
The intended logic is if sgx_inc_usage_count() returns nonzero,
the *incrementing of counter failed* (due to failed EUPDATESVN)
and we want to stop and report error.
  
> 
> But:
> 
> >  int sgx_inc_usage_count(void)
> >  {
> > +	int ret;
> > +
> > +	/*
> > +	 * Increments from non-zero indicate EPC other
> > +	 * active EPC users and EUPDATESVN is not attempted.
> > +	 */
> > +	if (atomic64_inc_not_zero(&sgx_usage_count))
> > +		return 0;
> 
> If sgx_usage_count is 1 here (ie. it's busy), this will return *zero*,
> and sgx_open() will not run into the -EBUSY condition and will continue
> assuming it has claimed the usage count, while it hasn't ...

Yes, meaning is different, see above. 

> 
> Furthermore, in the sgx_open() error paths we can then run into

What error paths? In case sgx_inc_usage_count() fails, we exit
immediately. 

> sgx_dec_usage_count(), which will merrily underflow the counter into
> negative:
> 
>  +void sgx_dec_usage_count(void)
>  +{
>  +       atomic64_dec(&sgx_usage_count);
>  +}
> 
> How is this all supposed to work?

Looks like I need more explanation on the counter and a better error
returned to userspace than -EBUSY. Maybe EIO then? 

Best Regards,
Elena.

> 
> Thanks,
> 
> 	Ingo