Re: [PATCH 2/4] x86/ibt: Implement FineIBT

[Kees Cook]

On Thu, Oct 27, 2022 at 11:28:14AM +0200, Peter Zijlstra wrote:
> Implement an alternative CFI scheme that merges both the fine-grained
> nature of kCFI but also takes full advantage of the coarse grained
> hardware CFI as provided by IBT.
> 
> To contrast:
> 
>   kCFI is a pure software CFI scheme and relies on being able to read
> text -- specifically the instruction *before* the target symbol, and
> does the hash validation *before* doing the call (otherwise control
> flow is compromised already).
> 
>   FineIBT is a software and hardware hybrid scheme; by ensuring every
> branch target starts with a hash validation it is possible to place
> the hash validation after the branch. This has several advantages:
> 
>    o the (hash) load is avoided; no memop; no RX requirement.
> 
>    o IBT WAIT-FOR-ENDBR state is a speculation stop; by placing
>      the hash validation in the immediate instruction after
>      the branch target there is a minimal speculation window
>      and the whole is a viable defence against SpectreBHB.
> 
>    o Kees feels obliged to mention it is slightly more vulnerable
>      when the attacker can write code.
> 
> Obviously this patch relies on kCFI, but additionally it also relies
> on the padding from the call-depth-tracking patches. It uses this
> padding to place the hash-validation while the call-sites are
> re-written to modify the indirect target to be 16 bytes in front of
> the original target, thus hitting this new preamble.
> 
> Notably, there is no hardware that needs call-depth-tracking (Skylake)
> and supports IBT (Tigerlake and onwards).
> 
> Suggested-by: Joao Moreira (Intel) <joao@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook