Re: [PATCH v3 09/10] x86/ibt: Implement FineIBT-BHI mitigation

[Kees Cook]

On Wed, Feb 19, 2025 at 05:21:16PM +0100, Peter Zijlstra wrote:
> While WAIT_FOR_ENDBR is specified to be a full speculation stop; it
> has been shown that some implementations are 'leaky' to such an extend
> that speculation can escape even the FineIBT preamble.
> 
> To deal with this, add additional hardening to the FineIBT preamble.
> 
> Notably, using a new LLVM feature:
> 
>   https://github.com/llvm/llvm-project/commit/e223485c9b38a5579991b8cebb6a200153eee245
> 
> which encodes the number of arguments in the kCFI preamble's register.
> 
> Using this register<->arity mapping, have the FineIBT preamble CALL
> into a stub clobbering the relevant argument registers in the
> speculative case.
> 
> (This is where Scott goes and gives more details...)
> 
> Suggested-by: Scott Constable <scott.d.constable@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Reviewed-by: Kees Cook <kees@xxxxxxxxxx>

-- 
Kees Cook