On Wed, Sep 26, 2018 at 1:55 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 09/26/2018 01:44 PM, Sean Christopherson wrote:
> > On Wed, Sep 26, 2018 at 01:16:59PM -0700, Dave Hansen wrote:
> >> We also need to clarify how this can happen. Is it through something
> >> than an app does, or is it solely when the hardware does something under
> >> the covers, like suspend/resume.
> >
> > Are you looking for something in the changelog, the comment, or just
> > a response? If it's the latter...
>
> Comments, please.
>
> > On bare metal with a bug-free kernel, the only scenario I'm aware of
> > where we'll encounter these faults is when hardware pulls the rug out
> > from under us. In a virtualized environment all bets are off because
> > the architecture allows VMMs to silently "destroy" the EPC at will,
> > e.g. KVM, and I believe Hyper-V, will take advantage of this behavior
> > to support live migration. Post migration, the destination system
> > will generate PF_SGX because the EPC{M} can't be migrated between
> > system, i.e. the destination EPCM sees all EPC pages as invalid.
>
> OK, cool.
>
> That's good background fodder for the changelog.
>
> But, for the comment, I'm happy with something like this:
>
> /*
> * The fault resulted from violation of SGX-specific access-
> * controls. This is expected to be the result of some lower
> * layer action (CPU suspend/resume, VM migration) and is
> * not related to anything the OS did. Treat it as an access
> * error to ensure it is passed up to the app via a signal where
> * it can be handled.
> */
>
> I really don't think we need to delve too deeply into the relationship
> between EPCM and PTEs or anything. Let's just say, "it's not the
> kernel's fault, it's not the app's fault, so throw up our hands".
There is a non-nitpicky consideration here. Logically, user code is
going to do this (totally made-up pseudocode):
enclave_t enclave = load_and_init_enclave(...);
int ret = sgx_run(enclave, some pointers to non-enclave-memory buffers, ...);
and, with the code in this patch, a correct implementation of
sgx_run() requires installing a signal handler. This is nasty, since
signal handlers, expecially for something like SIGSEGV or SIGBUS, are
not fantastic to say the least in libraries.
Could we perhaps have a little vDSO entry (or syscall, I suppose) that
runs an enclave an returns an error code, and rig up the #PF handler
to check if the error happened in the vDSO entry and fix it up rather
than sending a signal?
On Windows, this is much less of a concern, because Windows has real
scoped fault handling. But Linux doesn't, at least not yet.
--
Andy Lutomirski
AMA Capital Management, LLC
