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

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Message-ID: <202502191008.A13C96E1@keescook>
Date: Wed, 19 Feb 2025 10:11:52 -0800
From: Kees Cook <kees@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, alyssa.milburn@...el.com,
	scott.d.constable@...el.com, joao@...rdrivepizza.com,
	andrew.cooper3@...rix.com, jpoimboe@...nel.org,
	jose.marchesi@...cle.com, hjl.tools@...il.com,
	ndesaulniers@...gle.com, samitolvanen@...gle.com, nathan@...nel.org,
	ojeda@...nel.org, alexei.starovoitov@...il.com, mhiramat@...nel.org,
	jmill@....edu
Subject: Re: [PATCH v3 09/10] x86/ibt: Implement FineIBT-BHI mitigation

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@...el.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Reviewed-by: Kees Cook <kees@...nel.org>

-- 
Kees Cook