Re: [PATCH v39 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

[Jarkko Sakkinen]

On Tue, Oct 06, 2020 at 08:15:32AM -0700, Sean Christopherson wrote:
> On Tue, Oct 06, 2020 at 10:30:16AM +0200, Jethro Beekman wrote:
> > On 2020-10-06 04:57, Sean Christopherson wrote:
> > > On Sat, Oct 03, 2020 at 07:50:56AM +0300, Jarkko Sakkinen wrote:
> > >> +struct sgx_enclave_run {
> > >> +  __u64 tcs;
> > >> +  __u64 user_handler;
> > >> +  __u64 user_data;
> > >> +  __u32 leaf;
> > >
> > > I am still very strongly opposed to omitting exit_reason.  It is not at all
> > > difficult to imagine scenarios where 'leaf' alone is insufficient for the
> > > caller or its handler to deduce why the CPU exited the enclave.  E.g. see
> > > Jethro's request for intercepting interrupts.
> >
> > Not entirely sure what this has to do with my request, I just expect to see
> > leaf=ERESUME in this case, I think? E.g. as you would see in EAX when calling
> > ENCLU.
> 
> But how would you differentiate from the case that an exception occured in
> the enclave?  That will also transfer control with leaf=ERESUME.  If there
> was a prior exception and userspace didn't zero out the struct, there would
> be "valid" data in the exception fields.
> 
> An exit_reason also would allow retrofitting the exception fields into a
> union, i.e. the fields are valid if and only if exit_reason is exception.

Let's purge this a bit. Please remark where my logic goes wrong. I'm
just explaining how I've deduced the whole thing.

The information was encoded in v38 version of the vDSO was exactly this:

- On normal EEXIT, it got the value 0.
- Otherwise, it got the value 1.

The leaf, then embdded to struct sgx_exception but essentially the same
field got the value from EAX, and the value that EAX had was only
written on exception path.

Thus, I deduced that if you write $EEXIT to leaf on synchrous exit you
get the same information content, nothing gets overwritten. I.e. you
can make same conclusions as you would with those two struct fields.

/Jarkko