On Tue, Nov 6, 2018 at 4:02 PM Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote: > > On Tue, Nov 06, 2018 at 03:39:48PM -0800, Andy Lutomirski wrote: > > On Tue, Nov 6, 2018 at 3:35 PM Sean Christopherson > > <sean.j.christopherson@xxxxxxxxx> wrote: > > > > > > On Tue, Nov 06, 2018 at 03:00:56PM -0800, Andy Lutomirski wrote: > > > > > > > > > > > > >> On Nov 6, 2018, at 1:59 PM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote: > > > > >> > > > > >>> On Tue, 2018-11-06 at 13:41 -0800, Andy Lutomirski wrote: > > > > >> Sean, how does the current SDK AEX handler decide whether to do > > > > >> EENTER, ERESUME, or just bail and consider the enclave dead? It seems > > > > >> like the *CPU* could give a big hint, but I don't see where there is > > > > >> any architectural indication of why the AEX code got called or any > > > > >> obvious way for the user code to know whether the exit was fixed up by > > > > >> the kernel? > > > > > > > > > > The SDK "unconditionally" does ERESUME at the AEP location, but that's > > > > > bit misleading because its signal handler may muck with the context's > > > > > RIP, e.g. to abort the enclave on a fatal fault. > > > > > > > > > > On an event/exception from within an enclave, the event is immediately > > > > > delivered after loading synthetic state and changing RIP to the AEP. > > > > > In other words, jamming CPU state is essentially a bunch of vectoring > > > > > ucode preamble, but from software's perspective it's a normal event > > > > > that happens to point at the AEP instead of somewhere in the enclave. > > > > > And because the signals the SDK cares about are all synchronous, the > > > > > SDK can simply hardcode ERESUME at the AEP since all of the fault logic > > > > > resides in its signal handler. IRQs and whatnot simply trampoline back > > > > > into the enclave. > > > > > > > > > > Userspace can do something funky instead of ERESUME, but only *after* > > > > > IRET/RSM/VMRESUME has returned to the AEP location, and in Linux's > > > > > case, after the trap handler has run. > > > > > > > > > > Jumping back a bit, how much do we care about preventing userspace > > > > > from doing stupid things? > > > > > > > > My general feeling is that userspace should be allowed to do apparently > > > > stupid things. For example, as far as the kernel is concerned, Wine and > > > > DOSEMU are just user programs that do stupid things. Linux generally tries > > > > to provide a reasonably complete view of architectural behavior. This is > > > > in contrast to, say, Windows, where IIUC doing an unapproved WRFSBASE May > > > > cause very odd behavior indeed. So magic fixups that do non-architectural > > > > things are not so great. > > > > > > Sorry if I'm beating a dead horse, but what if we only did fixup on ENCLU > > > with a specific (ignored) prefix pattern? I.e. effectively make the magic > > > fixup opt-in, falling back to signals. Jamming RIP to skip ENCLU isn't > > > that far off the architecture, e.g. EENTER stuffs RCX with the next RIP so > > > that the enclave can EEXIT to immediately after the EENTER location. > > > > > > > How does that even work, though? On an AEX, RIP points to the ERESUME > > instruction, not the EENTER instruction, so if we skip it we just end > > up in lala land. > > Userspace would obviously need to be aware of the fixup behavior, but > it actually works out fairly nicely to have a separate path for ERESUME > fixup since a fault on EENTER is generally fatal, whereas as a fault on > ERESUME might be recoverable. > Hmm. > > do_eenter: > mov tcs, %rbx > lea async_exit, %rcx > mov $EENTER, %rax > ENCLU Or SOME_SILLY_PREFIX ENCLU? > > /* > * EEXIT or EENTER faulted. In the latter case, %RAX already holds some > * fault indicator, e.g. -EFAULT. > */ > eexit_or_eenter_fault: > ret But userspace wants to know whether it was a fault or not. So I think we either need two landing pads or we need to hijack a flag bit (are there any known-zeroed flag bits after EEXIT?) to say whether it was a fault. And, if it was a fault, we should give the vector, the sanitized error code, and possibly CR2. > > async_exit: > ENCLU Same prefix here, right? > > fixup_handler: > <do fault stuff> This whole thing is a bit odd, but not necessarily a terrible idea. > > > How averse would everyone be to making enclave entry be a syscall? > > The user code would do sys_sgx_enter_enclave(), and the kernel would > > stash away the register state (vm86()-style), point RIP to the vDSO's > > ENCLU instruction, point RCX to another vDSO ENCLU instruction, and > > SYSRET. The trap handlers would understand what's going on and > > restore register state accordingly. > > Wouldn't that blast away any stack changes made by the enclave? Yes, but I was imagining that it would stash the registers into the struct host_state thing I made up :)
