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

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Message-ID: <DM8PR11MB575052556CC247B621082110E72BA@DM8PR11MB5750.namprd11.prod.outlook.com>
Date: Tue, 12 Aug 2025 04:56:27 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>
CC: "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
	"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>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "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 11:37 PM
> To: Reshetova, Elena <elena.reshetova@...el.com>; Hansen, Dave
> <dave.hansen@...el.com>
> Cc: linux-sgx@...r.kernel.org; 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>; seanjc@...gle.com; 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.
> 
> Btw, I noticed this when I was looking at:
> 
> 	WARN(sgx_usage_count != 1, "...");
> 
> in patch 4 and wondering why it's not "!= 0".
> 
> Please don't forget to update that when needed.

Yes, of course.