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Date:   Tue, 20 Jun 2023 14:55:10 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     x86@kernel.org, alyssa.milburn@linux.intel.com,
        linux-kernel@vger.kernel.org, samitolvanen@google.com,
        jpoimboe@kernel.org, joao@overdrivepizza.com,
        tim.c.chen@linux.intel.com
Subject: Re: [PATCH 2/2] x86/fineibt: Poison ENDBR at +0
Message-ID: <202306201454.0A2E875F@keescook>
References: <20230615193546.949657149@infradead.org>
 <20230615193722.194131053@infradead.org>
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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@intel.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Looks like a good work-around.

Acked-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook