Re: [PATCH v3 7/7] x86/boot: Check that there are no runtime relocations

[Kees Cook]

On Mon, Jun 29, 2020 at 06:11:59PM +0200, Ard Biesheuvel wrote:
> On Mon, 29 Jun 2020 at 18:09, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >
> > On Mon, Jun 29, 2020 at 10:09:28AM -0400, Arvind Sankar wrote:
> > > Add a linker script check that there are no runtime relocations, and
> > > remove the old one that tries to check via looking for specially-named
> > > sections in the object files.
> > >
> > > Drop the tests for -fPIE compiler option and -pie linker option, as they
> > > are available in all supported gcc and binutils versions (as well as
> > > clang and lld).
> > >
> > > Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
> > > Reviewed-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
> > > ---
> > >  arch/x86/boot/compressed/Makefile      | 28 +++-----------------------
> > >  arch/x86/boot/compressed/vmlinux.lds.S |  8 ++++++++
> > >  2 files changed, 11 insertions(+), 25 deletions(-)
> >
> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> >
> > question below ...
> >
> > > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> > > index a4a4a59a2628..a78510046eec 100644
> > > --- a/arch/x86/boot/compressed/vmlinux.lds.S
> > > +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> > > @@ -42,6 +42,12 @@ SECTIONS
> > >               *(.rodata.*)
> > >               _erodata = . ;
> > >       }
> > > +     .rel.dyn : {
> > > +             *(.rel.*)
> > > +     }
> > > +     .rela.dyn : {
> > > +             *(.rela.*)
> > > +     }
> > >       .got : {
> > >               *(.got)
> > >       }
> >
> > Should these be marked (INFO) as well?
> >
> 
> Given that sections marked as (INFO) will still be emitted into the
> ELF image, it does not really make a difference to do this for zero
> sized sections.

Oh, I misunderstood -- I though they were _not_ emitted; I see now what
you said was not allocated. So, disk space used for the .got.plt case,
but not memory space used. Sorry for the confusion!

-Kees

> 
> > > @@ -85,3 +91,5 @@ ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT en
> > >  #else
> > >  ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, "Unexpected GOT/PLT entries detected!")
> > >  #endif
> > > +
> > > +ASSERT(SIZEOF(.rel.dyn) == 0 && SIZEOF(.rela.dyn) == 0, "Unexpected runtime relocations detected!")
> >
> > I think I should be doing this same ASSERT style for other explicit
> > DISCARDS in my orphan series so we'll notice if they change, instead
> > of being silently dropped if they grow.
> >

-- 
Kees Cook