linux-kernel - Re: [RFC PATCH 4/4] x86/vdso: Add __vdso_sgx_eenter() to wrap SGX enclave transitions

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Date:   Thu, 6 Dec 2018 06:17:18 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [RFC PATCH 4/4] x86/vdso: Add __vdso_sgx_eenter() to wrap SGX
 enclave transitions

On Thu, Dec 06, 2018 at 05:55:47AM -0800, Sean Christopherson wrote:
> On Wed, Dec 05, 2018 at 03:40:48PM -0800, Andy Lutomirski wrote:
> > On Wed, Dec 5, 2018 at 3:20 PM Sean Christopherson
> > <sean.j.christopherson@...el.com> wrote:
> > > +notrace long __vdso_sgx_eenter(void *tcs, void *priv,
> > > +                              struct sgx_eenter_fault_info *fault_info)
> > > +{
> > > +       u32 trapnr, error_code;
> > > +       long leaf;
> > > +       u64 addr;
> > > +
> > > +       /*
> > > +        *      %eax = EENTER
> > > +        *      %rbx = tcs
> > > +        *      %rcx = do_eresume
> > > +        *      %rdi = priv
> > > +        * do_eenter:
> > > +        *      enclu
> > > +        *      jmp     out
> > > +        *
> > > +        * do_eresume:
> > > +        *      enclu
> > > +        *      ud2
> > 
> > Is the only reason for do_eresume to be different from do_eenter so
> > that you can do the ud2?
> 
> No, it was a holdover from doing fixup via a magic prefix in user code.
> The fixup could only skip the ENCLU and so a second ENCLU was needed to
> differentiate between EENTER and ERESUME.  The need for two ENCLUs got
> ingrained in my head.  I can't think of anything that will break if we
> use a single ENCLU.
> 
> > > +        *
> > > +        * out:
> > > +        *      <return to C code>
> > > +        *
> > > +        * fault_fixup:
> > > +        *      <extable loads RDI, DSI and RDX with fault info>
> > > +        *      jmp     out
> > > +        */
> > 
> > This has the IMO excellent property that it's extremely awkward to use
> > it for a model where the enclave is reentrant.  I think it's excellent
> > because reentrancy on the same enclave thread is just asking for
> > severe bugs.  Of course, I fully expect the SDK to emulate reentrancy,
> > but then it's 100% their problem :)  On the fiip side, it means that
> > you can't really recover from a reported fault, even if you want to,
> > because there's no way to ask for ERESUME.  So maybe the API should
> > allow that after all.
> 
> Doh.  The ability to do ERESUME is an explicit requirement from the SDK
> folks.  More code that I pulled from my userspace implementation and
> didn't revisit.

Is it ok to add a separate exported function for ERESUME?  ERESUME can't
explicitly pass anything to the enclave, i.e. doesn't need a @priv param.
A separate function is a little prettier, e.g.:

static inline
long vdso_enter_enclave(enum sgx_enclu_leaf op, void *tcs, void *priv,
			struct sgx_eenter_fault_info *fault_info)
{
	...
}

notrace long __vdso_sgx_eenter(void *tcs, void *priv,
			       struct sgx_eenter_fault_info *fault_info)
{
	return vdso_enter_enclave(SGX_EENTER, tcs, priv, fault_info);
}

notrace long __vdso_sgx_eresume(void *tcs,
				struct sgx_eenter_fault_info *fault_info)
{
	return vdso_enter_enclave(SGX_ERESUME, tcs, NULL, fault_info);
}