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

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Message-ID: <c6f5356b-a56a-e057-ef74-74e1169a844b@intel.com>
Date:   Thu, 4 Nov 2021 08:29:49 -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 8:25 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.

> 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.