Re: [PATCH v5 29/36] x86/build: Enforce an empty .got.plt section

[Kees Cook]

On Fri, Jul 31, 2020 at 10:12:48PM -0400, Arvind Sankar wrote:
> On Fri, Jul 31, 2020 at 04:08:13PM -0700, Kees Cook wrote:
> > The .got.plt section should always be zero (or filled only with the
> > linker-generated lazy dispatch entry). Enforce this with an assert and
> > mark the section as NOLOAD. This is more sensitive than just blindly
> > discarding the section.
> > 
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >  arch/x86/kernel/vmlinux.lds.S | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> > index 0cc035cb15f1..7faffe7414d6 100644
> > --- a/arch/x86/kernel/vmlinux.lds.S
> > +++ b/arch/x86/kernel/vmlinux.lds.S
> > @@ -414,8 +414,20 @@ SECTIONS
> >  	ELF_DETAILS
> >  
> >  	DISCARDS
> > -}
> >  
> > +	/*
> > +	 * Make sure that the .got.plt is either completely empty or it
> > +	 * contains only the lazy dispatch entries.
> > +	 */
> > +	.got.plt (NOLOAD) : { *(.got.plt) }
> > +	ASSERT(SIZEOF(.got.plt) == 0 ||
> > +#ifdef CONFIG_X86_64
> > +	       SIZEOF(.got.plt) == 0x18,
> > +#else
> > +	       SIZEOF(.got.plt) == 0xc,
> > +#endif
> > +	       "Unexpected GOT/PLT entries detected!")
> > +}
> >  
> >  #ifdef CONFIG_X86_32
> >  /*
> > -- 
> > 2.25.1
> > 
> 
> Is this actually needed? vmlinux is a position-dependent executable, and
> it doesn't get linked with any shared libraries, so it should never have
> a .got or .got.plt at all I think? Does it show up as an orphan without
> this?

When I switched from DISCARD to 0-assert, I tested all of these, but given
so many combinations, perhaps I made a mistake. I will double-check and
report back.

-- 
Kees Cook