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

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Message-ID: <20200313192409.GA180706@linux.intel.com>
Date:   Fri, 13 Mar 2020 21:24:09 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Nathaniel McCallum <npmccallum@...hat.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-sgx@...r.kernel.org, akpm@...ux-foundation.org,
        dave.hansen@...el.com, Neil Horman <nhorman@...hat.com>,
        "Huang, Haitao" <haitao.huang@...el.com>,
        andriy.shevchenko@...ux.intel.com, tglx@...utronix.de,
        "Svahn, Kai" <kai.svahn@...el.com>, bp@...en8.de,
        Josh Triplett <josh@...htriplett.org>, luto@...nel.org,
        kai.huang@...el.com, rientjes@...gle.com, cedric.xing@...el.com,
        Patrick Uiterwijk <puiterwijk@...hat.com>,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v28 14/22] selftests/x86: Add a selftest for SGX

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