linux-kernel - Re: [PATCH V2] x86/sgx: Fix free page accounting

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Message-ID: <YYyNeW28jqKwD0tF@agluck-desk2.amr.corp.intel.com>
Date:   Wed, 10 Nov 2021 19:26:49 -0800
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Reinette Chatre <reinette.chatre@...el.com>,
        dave.hansen@...ux.intel.com, tglx@...utronix.de, bp@...en8.de,
        mingo@...hat.com, linux-sgx@...r.kernel.org, x86@...nel.org,
        seanjc@...gle.com, hpa@...or.com, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH V2] x86/sgx: Fix free page accounting

On Thu, Nov 11, 2021 at 04:55:14AM +0200, Jarkko Sakkinen wrote:
> On Wed, 2021-11-10 at 10:51 -0800, Reinette Chatre wrote:
> > sgx_should_reclaim() would only succeed when sgx_nr_free_pages goes 
> > below the watermark. Once sgx_nr_free_pages becomes corrupted there is 
> > no clear way in which it can correct itself since it is only ever 
> > incremented or decremented.
> 
> So one scenario would be:
> 
> 1. CPU A does a READ of sgx_nr_free_pages.
> 2. CPU B does a READ of sgx_nr_free_pages.
> 3. CPU A does a STORE of sgx_nr_free_pages.
> 4. CPU B does a STORE of sgx_nr_free_pages.
> 
> ?
> 
> That does corrupt the value, yes, but I don't see anything like this
> in the commit message, so I'll have to check.
> 
> I think the commit message is lacking a concurrency scenario, and the
> current transcripts are a bit useless.

What about this part:

	With sgx_nr_free_pages accessed and modified from a few places
	it is essential to ensure that these accesses are done safely but
	this is not the case. sgx_nr_free_pages is read without any
	protection and updated with inconsistent protection by any one
	of the spin locks associated with the individual NUMA nodes.
	For example:

	      CPU_A                                 CPU_B
	      -----                                 -----
	 spin_lock(&nodeA->lock);              spin_lock(&nodeB->lock);
	 ...                                   ...
	 sgx_nr_free_pages--;  /* NOT SAFE */  sgx_nr_free_pages--;

	 spin_unlock(&nodeA->lock);            spin_unlock(&nodeB->lock);

Maybe you missed the "NOT SAFE" hidden in the middle of
the picture?

-Tony