Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 18, 2022 at 05:11:27PM -0700, Linus Torvalds wrote:
> On Mon, Jul 18, 2022 at 5:03 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > So it already only adds the pattern to things that have their address
> > taken, not all functions?
> >
> > If so, that's simple enough to sort out: don't do any RSB stack
> > adjustment for those thunks AT ALL.
> >
> > Because they should just then end up with a jump to the "real" target,
> > and that real target will do the RSB stack thing.
> 
> Put another way, let's say that you have a function that looks like this:
> 
>   int silly(void)
>   {
>        return 0;
>   }
> 
> and now you have two cases:
> 
>  - the "direct callable version" of that function looks exactly the
> way it always has looked, and gets the 16 bytes of padding for it, and
> the RSB counting can happen in that padding
> 
>  - the "somebody took the address of this function" creates code that
> has the hash marker before it, and has the hash check, and then does a
> "jmp silly" to actually jump to the real code.
> 
> So what the RSB counting does is just ignore that second case entirely
> as far as the RSB code generation goes. No need to have any padding
> for it at all, it has that (completely different) kCFI padding
> instead.
> 
> Instead, only the "real" silly function gets that RSB code, and the
> "jmp silly" from the kCFI thunk needs to be updated to point to the
> RSB thunk in front of it.
> 
> Yes, yes, it makes indirect calls slightly more expensive than direct
> calls (because that kCFI thing can't just fall through to the real
> thing), but considering all the *other* costs of indirect calls, the
> cost of having that one "jmp" instruction doesn't really seem to
> matter, does it?

So it's like 2:15 am here, so I might not be following things right, but
doesn't the above mean you have to play funny games with what a function
pointer is?

That is, the content of a function pointer (address taken) no longer
match the actual function? That gives grief with things like
static_call(), ftrace and other things that write call instructions
instead of doing indirect calls.