linux-kernel - Re: [PATCH] x86/sgx: Free backing memory after faulting the enclave page

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Message-ID: <YYgsL7xSxnsjqIlu@iki.fi>
Date:   Sun, 7 Nov 2021 21:42:39 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Sean Christopherson <seanjc@...gle.com>,
        reinette.chatre@...el.com, tony.luck@...el.com,
        nathaniel@...fian.com, stable@...r.kernel.org,
        Borislav Petkov <bp@...e.de>, linux-sgx@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/sgx: Free backing memory after faulting the enclave
 page

On Thu, Nov 04, 2021 at 08:29:49AM -0700, Dave Hansen wrote:
> On 11/4/21 8:25 AM, Jarkko Sakkinen wrote:
> > On Thu, 2021-11-04 at 08:13 -0700, Dave Hansen wrote:
> >> On 11/4/21 8:04 AM, Jarkko Sakkinen wrote:
> >>>> Do we also need to deal with truncating the PCMD?  (For those watching
> >>>> along at home, there are two things SGX swaps to RAM: the actual page
> >>>> data and also some metadata that ensures page integrity and helps
> >>>> prevent things like rolling back to old versions of swapped pages)
> >>> Yes.
> >>>
> >>> This can be achieved by iterating through all of the enclave pages,
> >>> which share the same shmem page for storing their PCMD's, as the one
> >>> being faulted back. If none of those pages is swapped, the PCMD page can
> >>> safely truncated.
> >> I was thinking we could just read the page.  If it's all 0's, truncate it.
> > Hmm... did ELDU zero PCMD as a side-effect?
> 
> I don't think so, but there's nothing stopping us from doing it ourselves.

Ok.

> > It should be fairly effecient just to check the pages by using
> > encl->page_tree.
> 
> That sounds more complicated and slower than what I suggested.  You
> could even just check the refcount on the page.  I _think_ page cache
> pages have a refcount of 2.  So, look for the refcount that means "no
> more PCMD in this page", and just free it if so.

Umh, so... there is total 32 PCMD's per one page.

/Jarkko