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

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Wed, Mar 11, 2020 at 09:40:47AM -0700, Sean Christopherson wrote:
> 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.

Makes sense.

/Jarkko