On Tue, Apr 23, 2019 at 11:59 AM Sean Christopherson <sean.j.christopher...@intel.com> 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.
