Re: [PATCH] x86/ibt: Implement FineIBT

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Tue, Oct 18, 2022 at 03:35:50PM +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.
> 
> Obviously this patch relies on kCFI (upstream), but additionally it also
> relies on the padding from the call-depth-tracking patches
> (tip/x86/core). 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.

Can the objtool changes be moved to a separate patch?

The RFC was 11 patches, is it now much smaller because of the new
dependencies?  The RFC had some eBPF changes and a test module, are
those no longer needed?

-- 
Josh