Re: RFC: userspace exception fixups

[Jarkko Sakkinen]

On Sun, Nov 18, 2018 at 09:15:48AM +0200, Jarkko Sakkinen wrote:
> On Thu, Nov 01, 2018 at 10:53:40AM -0700, Andy Lutomirski wrote:
> > Hi all-
> > 
> > The people working on SGX enablement are grappling with a somewhat
> > annoying issue: the x86 EENTER instruction is used from user code and
> > can, as part of its normal-ish operation, raise an exception.  It is
> > also highly likely to be used from a library, and signal handling in
> > libraries is unpleasant at best.
> > 
> > There's been some discussion of adding a vDSO entry point to wrap
> > EENTER and do something sensible with the exceptions, but I'm
> > wondering if a more general mechanism would be helpful.
> 
> I haven't really followed all of this discussion because I've been busy
> working on the patch set but for me all of these approaches look awfully
> complicated.
> 
> I'll throw my own suggestion and apologize if this has been already
> suggested and discarded: return-to-AEP.
> 
> My idea is to do just a small extension to SGX AEX handling. At the
> moment hardware will RAX, RBX and RCX with ERESUME parameters. We can
> fill extend this by filling other three spare registers with exception
> information.
> 
> AEP handler can then do whatever it wants to do with this information
> or just do ERESUME.

A correction here. In practice this will add a requirement to have a bit
more complicated AEP code (check the regs for exceptions) than before
and not just bytes for ENCLU.

e.g. AEP handler should be along the lines

1. #PF (or #UD or) happens. Kernel fills the registers when it cannot
   handle the exception and returns back to user space i.e. to the
   AEP handler.
2. Check the registers containing exception information. If they have
   been filled, take whatever actions user space wants to take.
3. Otherwise, just ERESUME.

>From my point of view this is making the AEP parameter useful. Its
standard use is just weird (always point to a place just containing
ENCLU bytes, why the heck it even exists).

/Jarkko