Re: [RFC PATCH v1 3/3] selftests/x86: Augment SGX selftest to test new __vdso_sgx_enter_enclave() and its callback interface

[Andy Lutomirski]

On Tue, Apr 23, 2019 at 11:59 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Mon, Apr 22, 2019 at 06:29:06PM -0700, Andy Lutomirski wrote:
> > What's not tested here is running this code with EFLAGS.TF set and
> > making sure that it unwinds correctly.  Also, Jarkko, unless I missed
> > something, the vDSO extable code likely has a bug.  If you run the
> > instruction right before ENCLU with EFLAGS.TF set, then do_debug()
> > will eat the SIGTRAP and skip to the exception handler.  Similarly, if
> > you put an instruction breakpoint on ENCLU, it'll get skipped.  Or is
> > the code actually correct and am I just remembering wrong?
>
> The code is indeed broken, and I don't see a sane way to make it not
> broken other than to never do vDSO fixup on #DB or #BP.  But that's
> probably the right thing to do anyways since an attached debugger is
> likely the intended recipient the 99.9999999% of the time.
>
> The crux of the matter is that it's impossible to identify whether or
> not a #DB/#BP originated from within an enclave, e.g. an INT3 in an
> enclave will look identical to an INT3 at the AEP.  Even if hardware
> provided a magic flag, #DB still has scenarios where the intended
> recipient is ambiguous, e.g. data breakpoint encountered in the enclave
> but on an address outside of the enclave, breakpoint encountered in the
> enclave and a code breakpoint on the AEP, etc...

Ugh.  It sounds like ignoring the fixup for #DB is the right call.
But what happens if the enclave contains an INT3 or ICEBP instruction?
 Are they magically promoted to #GP, perhaps?

As a maybe possible alternative, if we made it so that the AEX address
was not the same as the ENCLU, could we usefully distinguish these
exceptions based on RIP?  I suppose it's also worth considering
whether page faults from *inside* the enclave should result in SIGSEGV
or result in a fixup.  We certainly want page faults from the ENCLU
instruction itself to get fixed up, but maybe we want most exceptions
inside the enclave to work a bit differently.  Of course, if we do
this, we need to make sure that the semantics of returning from the
signal handler are reasonable.