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

[Sami Tolvanen]

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@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > So it already only adds the pattern to things that have their address
> > taken, not all functions?

The preamble is added to address-taken static functions and all global
functions, because those might be indirectly called from other
translation units. With LTO, we could prune unnecessary preambles from
non-address-taken globals too.

> > 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.

Clang's current CFI implementation is somewhat similar to this. It
creates separate thunks for address-taken functions and changes
function addresses in C code to point to the thunks instead.

While this works, it creates painful situations when interacting with
assembly (e.g. a function address taken in assembly cannot be used
for indirect calls in C as it doesn't point to the thunk) and needs
unpleasant hacks when we want take the actual function address in C
(i.e. scattering the code with function_nocfi() calls).

I have to agree with Peter on this, I would rather avoid messing with
function pointers in KCFI to avoid these issues.

Sami