linux-kernel - Re: [PATCH v3 08/12] asm-generic: Refactor dereference_[kernel]_function

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Date:   Thu, 10 Feb 2022 16:56:55 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-ia64@...r.kernel.org, linux-parisc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 08/12] asm-generic: Refactor
 dereference_[kernel]_function_descriptor()

On Thu, Feb 10, 2022 at 09:30:43PM +1100, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@...roup.eu> writes:
> > diff --git a/kernel/extable.c b/kernel/extable.c
> > index b0ea5eb0c3b4..1ef13789bea9 100644
> > --- a/kernel/extable.c
> > +++ b/kernel/extable.c
> > @@ -159,12 +160,32 @@ int kernel_text_address(unsigned long addr)
> >  }
> >  
> >  /*
> > - * On some architectures (PPC64, IA64) function pointers
> > + * On some architectures (PPC64, IA64, PARISC) function pointers
> >   * are actually only tokens to some data that then holds the
> >   * real function address. As a result, to find if a function
> >   * pointer is part of the kernel text, we need to do some
> >   * special dereferencing first.
> >   */
> > +#ifdef CONFIG_HAVE_FUNCTION_DESCRIPTORS
> > +void *dereference_function_descriptor(void *ptr)
> > +{
> > +	func_desc_t *desc = ptr;
> > +	void *p;
> > +
> > +	if (!get_kernel_nofault(p, (void *)&desc->addr))
> > +		ptr = p;
> > +	return ptr;
> > +}
> 
> This needs an EXPORT_SYMBOL_GPL(), otherwise the build breaks after
> patch 10 with CONFIG_LKDTM=m.

Oh good catch!

(There have been a few cases of LKDTM=m being the only thing needed a
symbol, so I've pondered giving it a namespace or constructing a little
ifdef wrapper... but this seems ok to export...)

-- 
Kees Cook