Re: [PATCH 2/2] x86/fineibt: Poison ENDBR at +0

[Kees Cook]

On Thu, Jun 15, 2023 at 09:35:48PM +0200, Peter Zijlstra wrote:
> Alyssa noticed that when building the kernel with CFI_CLANG+IBT and
> booting on IBT enabled hardware obtain FineIBT, the indirect functions
> look like:
> 
>   __cfi_foo:
> 	endbr64
> 	subl	$hash, %r10d
> 	jz	1f
> 	ud2
> 	nop
>   1:
>   foo:
> 	endbr64
> 
> This is because clang currently does not supress ENDBR emission for
> functions it provides a __cfi prologue symbol for.

Should this be considered a bug in Clang?

> 
> Having this second ENDBR however makes it possible to elide the CFI
> check. Therefore, we should poison this second ENDBR (if present) when
> switching to FineIBT mode.
> 
> Fixes: 931ab63664f0 ("x86/ibt: Implement FineIBT")
> Reported-by: "Milburn, Alyssa" <alyssa.milburn@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Looks like a good work-around.

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook