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

[Sean Christopherson]

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...