linux-kernel - Re: [PATCH 1/2] x86/cfi: Fix ret_from

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Message-ID: <202306201455.AF16F617A@keescook>
Date:   Tue, 20 Jun 2023 14:56:22 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, alyssa.milburn@...ux.intel.com,
        linux-kernel@...r.kernel.org, samitolvanen@...gle.com,
        jpoimboe@...nel.org, joao@...rdrivepizza.com,
        tim.c.chen@...ux.intel.com
Subject: Re: [PATCH 1/2] x86/cfi: Fix ret_from_fork indirect calls

On Thu, Jun 15, 2023 at 09:35:47PM +0200, Peter Zijlstra wrote:
> The ret_from_fork stub does an indirect call to the kthread function,
> but only knows about Retpolines. Instead of making the asm more
> complicated, punt to C and let the compiler figure it out.
> 
> Specifically, this makes it a proper kCFI indirect call when needed (in
> fact, it is nearly impossible to code a kCFI indirect call in asm).
> 
> This was the only callsite that was still calling func()+0 on regular
> indirect functions.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

I worry this creates a calling gadget, but I don't think it really
counts since it's just converting between two prototypes. Regardless:

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

-- 
Kees Cook