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SubjectRe: [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@redhat.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.

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