linux-kernel - Re: RFC: userspace exception fixups

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Date:   Tue, 6 Nov 2018 15:26:16 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Rich Felker <dalias@...c.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Florian Weimer <fweimer@...hat.com>,
        Linux API <linux-api@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>, nhorman@...hat.com,
        npmccallum@...hat.com, "Ayoun, Serge" <serge.ayoun@...el.com>,
        shay.katz-zamir@...el.com, linux-sgx@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Carlos O'Donell <carlos@...hat.com>,
        adhemerval.zanella@...aro.org
Subject: Re: RFC: userspace exception fixups

On Tue, Nov 06, 2018 at 06:17:30PM -0500, Rich Felker wrote:
> On Tue, Nov 06, 2018 at 11:02:11AM -0800, Andy Lutomirski wrote:
> > On Tue, Nov 6, 2018 at 10:41 AM Dave Hansen <dave.hansen@...el.com> wrote:
> > >
> > > On 11/6/18 10:20 AM, Andy Lutomirski wrote:
> > > > I almost feel like the right solution is to call into SGX on its own
> > > > private stack or maybe even its own private address space.
> > >
> > > Yeah, I had the same gut feeling.  Couldn't the debugger even treat the
> > > enclave like its own "thread" with its own stack and its own set of
> > > registers and context?  That seems like a much more workable model than
> > > trying to weave it together with the EENTER context.
> > 
> > So maybe the API should be, roughly
> > 
> > sgx_exit_reason_t sgx_enter_enclave(pointer_to_enclave, struct
> > host_state *state);
> > sgx_exit_reason_t sgx_resume_enclave(same args);
> > 
> > where host_state is something like:
> > 
> > struct host_state {
> >   unsigned long bp, sp, ax, bx, cx, dx, si, di;
> > };
> > 
> > and the values in host_state explicitly have nothing to do with the
> > actual host registers.  So, if you want to use the outcall mechanism,
> > you'd allocate some memory, point sp to that memory, call
> > sgx_enter_enclave(), and then read that memory to do the outcall.
> > 
> > Actually implementing this would be distinctly nontrivial, and would
> > almost certainly need some degree of kernel help to avoid an explosion
> > when a signal gets delivered while we have host_state.sp loaded into
> > the actual SP register.  Maybe rseq could help with this?
> > 
> > The ISA here is IMO not well thought through.
> 
> Maybe I'm mistaken about some fundamentals here, but my understanding
> of SGX is that the whole point is that the host application and the
> code running in the enclave are mutually adversarial towards one
> another. Do any or all of the proposed protocols here account for this
> and fully protect the host application from malicious code in the
> enclave? It seems that having control over the register file on exit
> from the enclave is fundamentally problematic but I assume there must
> be some way I'm missing that this is fixed up.

SGX provides protections for the enclave but not the other way around.
The kernel has all of its normal non-SGX protections in place, but the
enclave can certainly wreak havoc on its userspace process.  The basic
design idea is that the enclave is a specialized .so that gets extra
security protections but is still effectively part of the overall
application, e.g. it has full access to its host userspace process'
virtual memory.