Re: [PATCH v14 09/19] x86/mm: x86/sgx: Signal SEGV_SGXERR for #PFs w/ PF_SGX

[Andy Lutomirski]

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