Re: [RFC PATCH 01/11] x86: kernel FineIBT

[Peter Zijlstra]

On Tue, May 03, 2022 at 03:02:44PM -0700, Josh Poimboeuf wrote:

> I'm not really qualified to comment on this too directly since I haven't
> looked very much at the variations on FineIBT/CFI/KCFI, and what the
> protections and drawbacks are for each approach, and when it might even
> make sense to combine them for a "paranoid user".
> 
> Since we have multiple similar and possibly competing technologies being
> discussed, one thing I do want to warn against is that we as kernel
> developers tend to err on the side of giving people too many choices and
> combinations which *never* get used.

So I don't think there's going to be a user choice here. If there's
hardware support, FineIBT makes more sense. That also means that kCFI no
longer needs to worry about IBT.

If we do something like:


        kCFI                                            FineIBT

__cfi_\sym:                                     __cfi_\sym:
        endbr                           # 4             endbr                   # 4
        sub $hash, %r10                 # 7             sub $hash, %r10         # 7
        je \sym                         # 2             je \sym                 # 2
        ud2                             # 2             ud2                     # 2
\sym:                                           \sym:


caller:                                         caller:
        cmpl $hash, -8(%r11)            # 8             movl $hash, %r10d       # 6
        je 1f                           # 2             sub 15, %r11            # 4
        ud2                             # 2             call *%r11              # 3
1:      call __x86_indirect_thunk_r11   # 5             .nop 4                  # 4 (could even fix up r11 again)


Then, all that's required is a slight tweak to apply_retpolines() to
rewrite a little more text.

Note that this also does away with having to fix up the linker, since
all direct call will already point at \sym. It's just the IBT indirect
calls that need to frob the pointer in order to hit the ENDBR.

On top of that, we no longer have to special case the objtool
instruction decoder, the prelude are proper instructions now.