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SubjectRe: [PATCH v28 14/22] selftests/x86: Add a selftest for SGX
On Wed, Mar 11, 2020 at 03:28:32PM +0200, Jarkko Sakkinen wrote:
> On Thu, Mar 05, 2020 at 01:33:28PM +0200, Jarkko Sakkinen wrote:
> > On Wed, 2020-03-04 at 14:27 -0500, Nathaniel McCallum wrote:
> > > > > +xsave_area:
> > > > + .fill 1, 4, 0x037F # FCW
> > > > + .fill 5, 4, 0
> > > > + .fill 1, 4, 0x1F80 # MXCSR
> > > > + .fill 1, 4, 0xFFFF # MXCSR_MASK
> > > > + .fill 123, 4, 0
> > > > + .fill 1, 4, 0x80000000 # XCOMP_BV[63] = 1, compaction mode
> > > > + .fill 12, 4, 0
> > >
> > > I find this much more readable:
> >
> > And I always aim to get things more readable. Thank you.
> >
> > > xsave_area:
> > > # Legacy
> > > .fill 1, 4, 0x037F # FCW
> > > .fill 5, 4, 0
> > > .fill 1, 4, 0x1F80 # MXCSR
> > > .fill 1, 4, 0xFFFF # MXCSR_MASK
> > > .fill 60, 8, 0
> > >
> > > # Header
> > > .fill 1, 8, 0 # XSTATE_BV
> > > .fill 1, 8, 1 << 63 # XCOMP_BV (compaction mode)
> > > .fill 6, 8, 0
> > >
> > > Also, since people are likely to copy this code for their own
> > > enclaves, it would be helpful to document which flags are set in FCW
> > > and MXCSR.
> >
> > It was meant as a test program but I'd guess what you say is true
> > because it also might be the only alternative user space to Intel's
> > :-) And a great starting point if you want to do things from scratch.
> >
> > Because I meant it as a smoke test program for SGX, not everything is
> > too well documented but given the multipurpose use for that code I'll
> > make the improvements that you are suggesting.
>
> For FPU Control World (FCW), I think 0x037F is not the right value even
> if section 18.5 in the x86 SDM says that it is the initial value for it.
>
> I took that value from that section.
>
> The reason I think that there is an error in the SDM is that if you look
> at the section 8.1.5, you'll see that bit 6 is a reserved bit. Thus,
> does not make to set it on.
>
> I think the legit value ought to be 0x33F i.e. unset bit 6.

Bit 6 is reserved, but it's forced to '1' by the CPU.

Regardless, IMO it'd be better to drop this code entirely, it's all kinds
of wonky. The label says "xsave_area" and implies XSAVE state is being
loaded, but the code uses FXRSTOR, which will only load x86/MMX/XMM state,
i.e. the first 512 bytes of the so called xsave_area.

The test enclave doesn't touch state managed by XSAVE, let alone put
secrets into said state. I see no reason to bother purging anything.

> In any case check:
>
> https://raw.githubusercontent.com/jsakkine-intel/linux-sgx/master/tools/testing/selftests/x86/sgx/encl_bootstrap.S
>
> I.e. both have now a reference:
>
> 1. To the section that describes the default.
> 2. To the section that describes what the bits mean.
>
> /Jarkko

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