Re: [RFC PATCH v2 4/4] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions

[Sean Christopherson]

On Tue, Dec 11, 2018 at 12:04:15PM -0800, Andy Lutomirski wrote:
> On Tue, Dec 11, 2018 at 11:31 AM Sean Christopherson
> <sean.j.christopherson@xxxxxxxxx> wrote:
> >
> > On Fri, Dec 07, 2018 at 03:33:57PM -0800, Andy Lutomirski wrote:
> > > On Fri, Dec 7, 2018 at 1:26 PM Sean Christopherson
> > > <sean.j.christopherson@xxxxxxxxx> wrote:
> > > >
> > > > Running a checksum on the stack for every exit doesn't seem like it'd
> > > > be worth the effort, especially since this type of bug should be quite
> > > > rare, at least in production environments.
> > > >
> > > > If we want to pursue the checksum idea I think the easiest approach
> > > > would be to combine it with an exit_handler and do a simple check on
> > > > the handler.  It'd be minimal overhead in the fast path and would flag
> > > > cases where invoking exit_handle() would explode, while deferring all
> > > > other checks to the user.
> > >
> > > How about this variant?
> > >
> > > #define MAGIC 0xaaaabbbbccccddddul
> > > #define RETADDR_HASH ((unsigned long)__builtin_return_address(0) ^ MAGIC)
> > >
> > > void foo(void)
> > > {
> > >     volatile unsigned long hash = RETADDR_HASH;
> > >
> > >     /* placeholder for your actual code */
> > >     asm volatile ("nop");
> > >
> > >     if (hash != RETADDR_HASH)
> > >         asm volatile ("ud2");
> > > }
> > >
> > > But I have a real argument for dropping exit_handler: in this new age
> > > of Spectre, the indirect call is a retpoline, and it's therefore quite
> > > slow.
> >
> > Technically slower, but would the extra CALL+RET pair even be noticeable
> > in the grand scheme of SGX?
> 
> But it's CALL, CALL, MOV to overwrite return address, intentionally
> midpredicted RET, and RET because Spectre.  That whole sequence seems
> to be several tens of cycles, so it's a lot worse than just CALL+RET.
> Whether it's noticeable overall is a fair question, though.

I was thinking of the case where the handler re-entered the enclave vs.
leaving and re-calling the vDSO, which would be RET+CALL and some other
stuff.