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

[Jarkko Sakkinen]

On Mon, 2020-03-16 at 10:01 -0400, Nathaniel McCallum wrote:
> On Mon, Mar 16, 2020 at 9:56 AM Jarkko Sakkinen
> <jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote:
> > On Sun, 2020-03-15 at 13:53 -0400, Nathaniel McCallum wrote:
> > > On Sat, Mar 14, 2020 at 9:25 PM Jarkko Sakkinen
> > > <jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote:
> > > > On Wed, Mar 11, 2020 at 01:30:07PM -0400, Nathaniel McCallum wrote:
> > > > > Currently, the selftest has a wrapper around
> > > > > __vdso_sgx_enter_enclave() which preserves all x86-64 ABI callee-saved
> > > > > registers (CSRs), though it uses none of them. Then it calls this
> > > > > function which uses %rbx but preserves none of the CSRs. Then it jumps
> > > > > into an enclave which zeroes all these registers before returning.
> > > > > Thus:
> > > > > 
> > > > >   1. wrapper saves all CSRs
> > > > >   2. wrapper repositions stack arguments
> > > > >   3. __vdso_sgx_enter_enclave() modifies, but does not save %rbx
> > > > >   4. selftest zeros all CSRs
> > > > >   5. wrapper loads all CSRs
> > > > > 
> > > > > I'd like to propose instead that the enclave be responsible for saving
> > > > > and restoring CSRs. So instead of the above we have:
> > > > >   1. __vdso_sgx_enter_enclave() saves %rbx
> > > > >   2. enclave saves CSRs
> > > > >   3. enclave loads CSRs
> > > > >   4. __vdso_sgx_enter_enclave() loads %rbx
> > > > > 
> > > > > I know that lots of other stuff happens during enclave transitions,
> > > > > but at the very least we could reduce the number of instructions
> > > > > through this critical path.
> > > > 
> > > > What Jethro said and also that it is a good general principle to cut
> > > > down the semantics of any vdso as minimal as possible.
> > > > 
> > > > I.e. even if saving RBX would make somehow sense it *can* be left
> > > > out without loss in terms of what can be done with the vDSO.
> > > 
> > > Please read the rest of the thread. Sean and I have hammered out some
> > > sensible and effective changes.
> > 
> > Have skimmed through that discussion but it comes down how much you get
> > by obviously degrading some of the robustness. Complexity of the calling
> > pattern is not something that should be emphasized as that is something
> > that is anyway hidden inside the runtime.
> 
> 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. And since this has been kind of agreed by most of the
stakeholders doing something against the chosen strategy is
something I do hold some resistance.

I get the idea technically what you are suggesting. Please
understand these are orthogonal axes that I have to care about.

In coummunity sense, it opens a possibility to unknown unknowns [1].

[1] https://www.youtube.com/watch?v=GiPe1OiKQuk

/Jarkko