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

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Message-ID: <55eb8f3649590289a0f2b1ebe7583b6da3ff70ee.camel@kernel.org>
Date:   Thu, 04 Nov 2021 17:25:03 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>,
        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>
Cc:     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, 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?

It should be fairly effecient just to check the pages by using
encl->page_tree.

/Jarkko