Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call

[Nathaniel McCallum]

On Wed, Mar 18, 2020 at 9:01 AM Nathaniel McCallum
<npmccallum@xxxxxxxxxx> wrote:
>
> On Tue, Mar 17, 2020 at 6:23 PM Xing, Cedric <cedric.xing@xxxxxxxxx> wrote:
> >
> > On 3/17/2020 9:50 AM, Nathaniel McCallum wrote:
> > > On Mon, Mar 16, 2020 at 8:18 PM Xing, Cedric <cedric.xing@xxxxxxxxx> wrote:
> > >>
> > >> On 3/16/2020 4:59 PM, Sean Christopherson wrote:
> > >>> On Mon, Mar 16, 2020 at 04:50:26PM -0700, Xing, Cedric wrote:
> > >>>> On 3/16/2020 3:53 PM, Sean Christopherson wrote:
> > >>>>> On Mon, Mar 16, 2020 at 11:38:24PM +0200, Jarkko Sakkinen wrote:
> > >>>>>>> My suggestions explicitly maintained robustness, and in fact increased
> > >>>>>>> it. If you think we've lost capability, please speak with specificity
> > >>>>>>> rather than in vague generalities. Under my suggestions we can:
> > >>>>>>> 1. call the vDSO from C
> > >>>>>>> 2. pass context to the handler
> > >>>>>>> 3. have additional stack manipulation options in the handler
> > >>>>>>>
> > >>>>>>> The cost for this is a net 2 additional instructions. No existing
> > >>>>>>> capability is lost.
> > >>>>>>
> > >>>>>> My vague generality in this case is just that the whole design
> > >>>>>> approach so far has been to minimize the amount of wrapping to
> > >>>>>> EENTER.
> > >>>>>
> > >>>>> Yes and no.   If we wanted to minimize the amount of wrapping around the
> > >>>>> vDSO's ENCLU then we wouldn't have the exit handler shenanigans in the
> > >>>>> first place.  The whole process has been about balancing the wants of each
> > >>>>> use case against the overall quality of the API and code.
> > >>>>>
> > >>>> The design of this vDSO API was NOT to minimize wrapping, but to allow
> > >>>> maximal flexibility. More specifically, we strove not to restrict how info
> > >>>> was exchanged between the enclave and its host process. After all, calling
> > >>>> convention is compiler specific - i.e. the enclave could be built by a
> > >>>> different compiler (e.g. MSVC) that doesn't share the same list of CSRs as
> > >>>> the host process. Therefore, the API has been implemented to pass through
> > >>>> virtually all registers except those used by EENTER itself. Similarly, all
> > >>>> registers are passed back from enclave to the caller (or the exit handler)
> > >>>> except those used by EEXIT. %rbp is an exception because the vDSO API has to
> > >>>> anchor the stack, using either %rsp or %rbp. We picked %rbp to allow the
> > >>>> enclave to allocate space on the stack.
> > >>>
> > >>> And unless I'm missing something, using %rcx to pass @leaf would still
> > >>> satisfy the above, correct?  Ditto for saving/restoring %rbx.
> > >>>
> > >>> I.e. a runtime that's designed to work with enclave's using a different
> > >>> calling convention wouldn't be able to take advantage of being able to call
> > >>> the vDSO from C, but neither would it take on any meaningful burden.
> > >>>
> > >> Not exactly.
> > >>
> > >> If called directly from C code, the caller would expect CSRs to be
> > >> preserved.
> > >
> > > Correct. This requires collaboration between the caller of the vDSO
> > > and the enclave.
> > >
> > >> Then who should preserve CSRs?
> > >
> > > The enclave.
> > >
> > >> It can't be the enclave
> > >> because it may not follow the same calling convention.
> > >
> > > This is incorrect. You are presuming there is not tight integration
> > > between the caller of the vDSO and the enclave. In my case, the
> > > integration is total and complete. We have working code today that
> > > does this.
> > >
> > >> Moreover, the
> > >> enclave may run into an exception, in which case it doesn't have the
> > >> ability to restore CSRs.
> > >
> > > There are two solutions to this:
> > > 1. Write the handler in assembly and don't return to C on AEX.
> > > 2. The caller can simply preserve the registers. Nothing stops that.
> > >
> > > We have implemented #1.
> > >
> > What if the enclave cannot proceed due to an unhandled exception so the
> > execution has to get back to the C caller of the vDSO API?
>
> mov $60, %rax
> mov $1, %rdi
> syscall
>
> We exit in all such cases.

Another solution is for the enclave to push the non-volatile registers
to the non-enclave stack upon entry and let the handler restore them.
That works for both EEXIT and AEX and you can return to C code then.

> > It seems to me the caller has to preserve CSRs by itself, otherwise it
> > cannot continue execution after any enclave exception. Passing @leaf in
> > %ecx will allow saving/restoring CSRs in C by setjmp()/longjmp(), with
> > the help of an exit handler. But if the C caller has already preserved
> > CSRs, why preserve CSRs again inside the enclave? It looks to me things
> > can be simplified only if the host process handles no enclave exceptions
> > (or exceptions inside the enclave will crash the calling thread). Thus
> > the only case of enclave EEXIT'ing back to its caller is considered
> > valid, hence the enclave will always be able to restore CSRs, so that
> > neither vDSO nor its caller has to preserve CSRs.
> >
> > Is my understanding correct?
> >