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

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Message-ID: <202306211107.4D529222C@keescook>
Date:   Wed, 21 Jun 2023 11:08:46 -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 Wed, Jun 21, 2023 at 11:27:59AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 21, 2023 at 10:52:17AM +0200, Peter Zijlstra wrote:
> > 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.

Ah yeah, it should be direct-called only. I keep forgetting about the
endbr removal pass.

> I can't seem to manage to have it clobber it's __cfi hash value. Ideally
> we'd have an attribute to force the thing to 0 or something.

Doesn't objtool have logic to figure out this is only ever
direct-called?

-- 
Kees Cook