Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 7 Dec 2018 12:09:35 -0800 | From | Sean Christopherson <> | Subject | Re: [RFC PATCH v2 4/4] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions |
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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@intel.com> 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@intel.com> 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. You’re 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.
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