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

[Sean Christopherson]

On Sun, Apr 26, 2020 at 11:57:53AM -0500, Dr. Greg wrote:
> In closing, it is important to note that the proposed SGX driver is
> not available as a module.  This effectively excludes any alternative
> implementations of the driver without replacement of the kernel at
> large.

No it doesn't.  The SGX subsytem won't allocate EPC pages unless userspace
creates an enclave, i.e. preventing unprivileged userspace from accessing
/dev/sgx/enclave will allow loading an alternative out-of-tree SGX module.
Yes, SGX sanitizes the EPC on boot, but that's arguably a good thing for
out-of-tree modules.

And if you want to get crafty and squash in-kernel SGX altogether, boot
with "clearcpuid=<SGX_LC>" and/or "clearcpuid=<SGX>" to disable in-kernel
support entirely.  SGX won't be correctly enumerated in /proc/cpuinfo
relative to the existence of an out-of-tree module, but that seems like a
very minor issue if you're running with a completely different SGX driver.

> It also means that any platform, with SGX hardware support,
> running a kernel with this driver, has the potential for the
> security/privacy issues noted above.

Unless I'm mistaken, /dev/sgx is root-only by default.  There are far
scarier mechanisms available to root for hosing the system.

> If key based policy management is not allowed, then the driver needs
> to be re-architected to have modular support so that alternative
> implementations or the absence of any driver support are at least
> tenable.

As above, using an alternative implementation is teneble, albeit a bit
kludgy.