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

[Haitao Huang]

On Wed, 06 May 2020 17:14:22 -0500, Sean Christopherson  
<sean.j.christopherson@xxxxxxxxx> wrote:

> 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@xxxxxxxxxx>
> >
> > 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.

For larger size mmap, I think it requires enabling vm overcommit mode 1:
echo 1 | sudo tee /proc/sys/vm/overcommit_memory


> > 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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