Subject: Re: [PATCH v3 08/12] asm-generic: Refactor dereference_[kernel]_function_descriptor()

From: Kees Cook <[email-masked]>
Date: Thu, 10 Feb 2022 16:56:55 -0800

On Thu, Feb 10, 2022 at 09:30:43PM +1100, Michael Ellerman wrote:
> Christophe Leroy <[email-masked]> 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