linux-kernel - Re: [PATCH RESEND 0/3] x86/sgx: eextend ioctl

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Date:   Fri, 2 Apr 2021 11:42:53 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jethro Beekman <jethro@...tanix.com>,
        Raoul Strackx <raoul.strackx@...tanix.com>,
        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>,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND 0/3] x86/sgx: eextend ioctl

On 4/2/21 11:31 AM, Jethro Beekman wrote:
> On 2021-04-02 17:53, Dave Hansen wrote:
>> On 4/2/21 1:38 AM, Jethro Beekman wrote:
>>>> So, we're talking here about pages that have been EEADDED, but for
>>>> which we do not want to include the entire contents of the page?
>>>> Do these contents always include the beginning of the page, or can
>>>> the holes be anywhere?
>>> Holes can be anywhere, and EEXTEND calls need not be sequential in
>>> memory address or even relate to the most recently EADDed page.
>>
>> I think you're referring to the SGX architecture itself here.  The
>> architecture permits this, right?
> 
> Yes.
> 
>> But, why would an enclave loader application ever do this? 
> 
> e.g. to save space

How does this save space, exactly?  What space does it save?

Let's say I want to add 4352 bytes of data to an enclave.  Today, I have
to page-align the beginning and end of that 4352 bytes, and call the add
ioctl() to add the two resulting pages.  It consumes two EPC pages.

With EEXTEND, if I want to add the data, I only need to page-align the
beginning of the data.  I call add_page on the first page, then eextend
on the 256 bytes.  It consumes two EPC pages.

I guess you can argue that this saves padding out the second page, which
would *theoretically* temporarily eat up one extra page of non-enclave
memory and the cost of a 256-byte memcpy.

>> Is this something we want to support in Linux?
> 
> Why not? Is there a good reason to not fully support this part of the
> CPU architecture?

We don't blindly support CPU features in Linux.  They need to do
something *useful*.  I'm still missing what this does which is useful.

Does any actual, real-world enclave want this functionality?  Why?

P.S. There are plenty of things you can do with the SGX architecture
that we probably won't ever implement in Linux.