Re: [PATCH v28 14/22] selftests/x86: Add a selftest for SGX

[Sean Christopherson]

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