On Fri, Nov 2, 2018 at 11:04 PM Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote: > On Fri, Nov 02, 2018 at 08:02:23PM +0100, Jann Horn wrote: > > On Fri, Nov 2, 2018 at 7:27 PM Sean Christopherson > > <sean.j.christopherson@xxxxxxxxx> wrote: > > > On Fri, Nov 02, 2018 at 10:48:38AM -0700, Andy Lutomirski wrote: > > > > This whole mechanism seems very complicated, and it's not clear > > > > exactly what behavior user code wants. > > > > > > No argument there. That's why I like the approach of dumping the > > > exception to userspace without trying to do anything intelligent in > > > the kernel. Userspace can then do whatever it wants AND we don't > > > have to worry about mucking with stacks. > > > > > > One of the hiccups with the VDSO approach is that the enclave may > > > want to use the untrusted stack, i.e. the stack that has the VDSO's > > > stack frame. For example, Intel's SDK uses the untrusted stack to > > > pass parameters for EEXIT, which means an AEX might occur with what > > > is effectively a bad stack from the VDSO's perspective. > > > > What exactly does "uses the untrusted stack to pass parameters for > > EEXIT" mean? I guess you're saying that the enclave is writing to > > RSP+[0...some_positive_offset], and the written data needs to be > > visible to the code outside the enclave afterwards? > > As is, they actually do it the other way around, i.e. negative offsets > relative to the untrusted %RSP. Going into the enclave there is no > reserved space on the stack. The SDK uses EEXIT like a function call, > i.e. pushing parameters on the stack and making an call outside of the > enclave, hence the name out-call. This allows the SDK to handle any > reasonable out-call without a priori knowledge of the application's > maximum out-call "size". But presumably this is bounded to be at most 128 bytes (the red zone size), right? Otherwise this would be incompatible with non-sigaltstack signal delivery.
