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

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Date:   Thu, 4 Nov 2021 08:13:39 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>,
        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 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.