linux-kernel - Re: [RFC PATCH v2 4/4] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions

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Date:   Fri, 7 Dec 2018 11:02:57 -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>,
        linux-sgx@...r.kernel.org, haitao.huang@...ux.intel.com,
        Jethro Beekman <jethro@...tanix.com>,
        "Dr. Greg Wettstein" <greg@...ellic.com>
Subject: Re: [RFC PATCH v2 4/4] x86/vdso: Add __vdso_sgx_enter_enclave() to
 wrap SGX enclave transitions

On Fri, Dec 07, 2018 at 09:56:09AM -0800, Andy Lutomirski wrote:
> On Fri, Dec 7, 2018 at 8:51 AM Sean Christopherson
> <sean.j.christopherson@...el.com> wrote:
> > I like that the exit handler allows userspace to trap/panic with the full
> > call stack in place, and in a dedicated path, i.e. outside of the basic
> > enter/exit code.  An exit handler probably doesn't fundamentally change
> > what userspace can do with respect to debugging/reporting, but I think
> > it would actually simplify some userspace implementations, e.g. I'd use
> > it in my tests like so:
> >
> > long fault_handler(struct sgx_enclave_exit_info *exit_info, void *tcs, void *priv)
> > {
> >         if (exit_info->leaf == SGX_EEXIT)
> >                 return 0;
> >
> >         <report exception and die/hang>
> > }
> >
> 
> Hmm.  That't not totally silly, although you could accomplish almost
> the same thing by wrapping the vDSO helper in another function.

True, but I think there's value in having the option to intercept an
exception at the exact(ish) point of failure, without having to return
up the stack.

The enclave has full access to the process' memory space, including the
untrsuted stack.  It's not too far fetched to envision a scenario where
the enclave corrupts the stack even if the enclave isn't intentionally
using the stack, e.g. the host passes in variable that a points at the
stack instead of whatever memory is supposed to be shared between the
enclave and host.  It'd be nice to have the ability to triage something
like that without having to e.g. configure breakpoints on the stack.