linux-kernel - RE: [PATCH v12 5/5] x86/sgx: Enable automatic SVN updates for SGX enclaves

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Message-ID: <DM8PR11MB575066A9AF7D7055CBCF2EC5E728A@DM8PR11MB5750.namprd11.prod.outlook.com>
Date: Mon, 11 Aug 2025 12:14:29 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "mingo@...nel.org"
	<mingo@...nel.org>, "Scarlata, Vincent R" <vincent.r.scarlata@...el.com>,
	"x86@...nel.org" <x86@...nel.org>, "jarkko@...nel.org" <jarkko@...nel.org>,
	"Annapurve, Vishal" <vannapurve@...gle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Mallick, Asit K" <asit.k.mallick@...el.com>,
	"Aktas, Erdem" <erdemaktas@...gle.com>, "Cai, Chong" <chongc@...gle.com>,
	"Bondarevska, Nataliia" <bondarn@...gle.com>, "linux-sgx@...r.kernel.org"
	<linux-sgx@...r.kernel.org>, "Raynor, Scott" <scott.raynor@...el.com>
Subject: RE: [PATCH v12 5/5] x86/sgx: Enable automatic SVN updates for SGX
 enclaves



> -----Original Message-----
> From: Huang, Kai <kai.huang@...el.com>
> Sent: Monday, August 11, 2025 1:50 PM
> To: Reshetova, Elena <elena.reshetova@...el.com>; Hansen, Dave
> <dave.hansen@...el.com>
> Cc: seanjc@...gle.com; mingo@...nel.org; Scarlata, Vincent R
> <vincent.r.scarlata@...el.com>; x86@...nel.org; jarkko@...nel.org;
> Annapurve, Vishal <vannapurve@...gle.com>; linux-kernel@...r.kernel.org;
> Mallick, Asit K <asit.k.mallick@...el.com>; Aktas, Erdem
> <erdemaktas@...gle.com>; Cai, Chong <chongc@...gle.com>; Bondarevska,
> Nataliia <bondarn@...gle.com>; linux-sgx@...r.kernel.org; Raynor, Scott
> <scott.raynor@...el.com>
> Subject: Re: [PATCH v12 5/5] x86/sgx: Enable automatic SVN updates for SGX
> enclaves
> 
> 
> >
> > +/* Mutex to ensure no concurrent EPC accesses during EUPDATESVN */
> > +static DEFINE_MUTEX(sgx_svn_lock);
> > +
> >  int sgx_inc_usage_count(void)
> >  {
> > +	guard(mutex)(&sgx_svn_lock);
> > +
> > +	if (sgx_usage_count++ == 0)
> > +		return sgx_update_svn();
> > +
> 
> Hmm.. sorry for not noticing this before.. But I think we might have a
> problem here since the sgx_usage_count is increased regardless of the
> result of sgx_update_svn().
> 
> If sgx_update_svn() fails, it makes sgx_inc_usage_count() return error
> too, so sgx_{vepc_}open() will fail and return immediately w/o calling
> sgx_dec_usage_count().
> 
> But the sgx_usage_count has been increased.
> 
> AFAICT when sgx_{vepc_}_open() fails, the sgx_{vepc_}release() is not
> called, so sgx_dec_usage_count() is never called and sgx_usage_count
> remains increased.
> 
> So when sgx_{vepc_}open() calls sgx_inc_usage_count() again, it will skip
> calling sgx_update_svn(), and allow enclave/vEPC to be created
> successfully, which just defeats the purpose.
> 
> So if I am not missing anything, I think we should only increase the count
> when sgx_update_svn() returns success?

Yes, you are right, thanks for catching this! In past the atomic version of
this patch did exactly, but after I went into this simplified version of counting,
this angle got broken.
Will fix. 

Best Regards,
Elena.