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

[Sean Christopherson]

On Fri, Dec 07, 2018 at 11:23:10AM -0800, Andy Lutomirski wrote:
> 
> > On Dec 7, 2018, at 11:02 AM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> > 
> >> 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@xxxxxxxxx> 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.
> 
> Ah, I see. Youâre saying that, if the non-enclave stare is corrupted such
> that RIP  is okay and RSP still points somewhere reasonable but the return
> address is garbage, then we can at least get to the fault handler and print
> something?

Yep.  Even for something more subtle like GPR corruption it could dump the
entire call stack before attempting to return back up.

> This only works if the fault handler pointer itself is okay, though, which
> somewhat limits the usefulness, given that its pointer is quite likely to
> be on the stack very close to the return address.

Yeah, it's not a silver bullet by any means, but it does seem useful for at
least some scenarios.  Even exploding when invoking the handler instead of
at a random point might prove useful, e.g. "calling my exit handler exploded,
maybe my enclave corrupted the stack!".
 
> I really wish the ENCLU instruction had seen fit to preserve some registers.

Speaking of preserving registers, the asm blob needs to mark RBX as
clobbered since it's modified for EEXIT.