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

[Sean Christopherson <sean.j.christopherson@intel.com>]

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@intel.com> 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?

> So I'm not saying NAK, but I do think it's unnecessary.

Ya, definitely not necessary, but it does allow userspace do things that
are otherwise cumbersome or impossible to do with the vanilla vDSO.  How
much value that actually adds is another question...