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

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Date:   Wed, 21 Jun 2023 10:52:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.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 Tue, Jun 20, 2023 at 02:56:22PM -0700, Kees Cook wrote:
> 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:

Ah, since this will never be indirectly called, I should be able to
annotate this so it never can be. Let me see what I can get the compiler
to do.