linux-kernel - Re: [PATCH v29 00/20] Intel SGX foundations

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Date:   Wed, 6 May 2020 15:14:22 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Nathaniel McCallum <npmccallum@...hat.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-sgx@...r.kernel.org, akpm@...ux-foundation.org,
        dave.hansen@...el.com, Neil Horman <nhorman@...hat.com>,
        "Huang, Haitao" <haitao.huang@...el.com>,
        andriy.shevchenko@...ux.intel.com, tglx@...utronix.de,
        "Svahn, Kai" <kai.svahn@...el.com>, bp@...en8.de,
        Josh Triplett <josh@...htriplett.org>, luto@...nel.org,
        kai.huang@...el.com, David Rientjes <rientjes@...gle.com>,
        "Xing, Cedric" <cedric.xing@...el.com>,
        Patrick Uiterwijk <puiterwijk@...hat.com>
Subject: Re: [PATCH v29 00/20] Intel SGX foundations

On Wed, May 06, 2020 at 05:42:42PM -0400, Nathaniel McCallum wrote:
> Tested on Enarx. This requires a patch[0] for v29 support.
> 
> Tested-by: Nathaniel McCallum <npmccallum@...hat.com>
> 
> However, we did uncover a small usability issue. See below.
> 
> [0]: https://github.com/enarx/enarx/pull/507/commits/80da2352aba46aa7bc6b4d1fccf20fe1bda58662

...

> > * Disallow mmap(PROT_NONE) from /dev/sgx. Any mapping (e.g. anonymous) can
> >   be used to reserve the address range. Now /dev/sgx supports only opaque
> >   mappings to the (initialized) enclave data.
> 
> The statement "Any mapping..." isn't actually true.
> 
> Enarx creates a large enclave (currently 64GiB). This worked when we
> created a file-backed mapping on /dev/sgx/enclave. However, switching
> to an anonymous mapping fails with ENOMEM. We suspect this is because
> the kernel attempts to allocate all the pages and zero them but there
> is insufficient RAM available. We currently work around this by
> creating a shared mapping on /dev/zero.

Hmm, the kernel shouldn't actually allocate physical pages unless they're
written.  I'll see if I can reproduce.
 
> If we want to keep this mmap() strategy, we probably don't want to
> advise mmap(ANON) if it allocates all the memory for the enclave ahead
> of time, even if it won't be used. This would be wasteful.
> 
> OTOH, having to mmap("/dev/zero") seems a bit awkward.