On Wed, Mar 20, 2019 at 01:38:04PM -0700, Xing, Cedric wrote: > > > By requiring preservation of RSP at both AEX and EEXIT, this precludes > > > the possibility of using the untrusted stack as temporary storage by > > > enclaves. While that looks reasonable at first glance, I'm afraid it > > > isn't the case in reality. The untrusted stack is inarguably the most > > > convenient way for data exchange between an enclave and its enclosing > > > process, > > > > I vehemently disagree with "inarguably". IMO, passing data via > > registers is much more convenient. > > Which is the most convenient approach is always dependent on data size and/or even how the data is produced/consumed. It's kind of a spectrum and we're just talking in the sense of probability. You are right that "inarguably" is arguable if the buffer is small enough to fit in registers, and the producer/consumer also has access to registers. > > > > > Even if you qualify your assertion with "data of arbitrary size unknown > > at build time", I still disagree. Using the untrusted stack allows for > > trickery when a debugger is involved, other than that I see no > > advantages over allocating virtual memory and handing the pointer to the > > enclave at launch time. Sure, it requires a few more lines of code to > > setup, but it's literally ~20 LoC out of thousands required to sign, > > build and launch an enclave, but it doesn't require playing games with > > the stack. > > I'm NOT ruling out your approach. > > And like you said, the untrusted stack enables certain trickery that helps > debugging and also simplifies enclaves (even just a little). Then why are > you trying to rule that out? Because of 9 LOC in vDSO? Although its just 9 LOC, consider its impact on someone who is looking at the kernel's SGX support for the first time. Questions they may have when looking at the vDSO code/documentation: - What's an exit handler? - Why is an exit handler optional? Don't I always want to handle exits? - What value should my exit handler return? - What should my exit handler do if it detects an error? - Why would I want to preserve %rbp and not %rsp? - Isn't it insecure to use the untrusted stack in my enclave? AFAIK, the only reason to preserve %rbp instead of %rsp, i.e. support an "exit handler" callback, is to be able to implement an o-call scheme using the untrusted stack to pass data. Every idea I came up with for using the callback, e.g. logging, handling stack corruptiong, testing hooks, etc... was at worst no more difficult to implement when using a barebones vDSO. So, given the choice between a) documenting and maintaining all the baggage that comes with the exit handler and b) saying "go use signals", I chose option b.
