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

[Ard Biesheuvel <ardb@kernel.org>]

On Tue, 30 Jun 2020 at 01:34, Fangrui Song <maskray@google.com> wrote:
>
> On 2020-06-29, Ard Biesheuvel wrote:
> >On Mon, 29 Jun 2020 at 19:37, Fangrui Song <maskray@google.com> wrote:
> >>
> >> On 2020-06-29, Arvind Sankar wrote:
> >> >On Mon, Jun 29, 2020 at 09:20:31AM -0700, Kees Cook wrote:
> >> >> On Mon, Jun 29, 2020 at 06:11:59PM +0200, Ard Biesheuvel wrote:
> >> >> > On Mon, 29 Jun 2020 at 18:09, Kees Cook <keescook@chromium.org> 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@alum.mit.edu>
> >> >> > > > Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
> >> >> > > > Reviewed-by: Fangrui Song <maskray@google.com>
> >> >> > > > ---
> >> >> > > >  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@chromium.org>
> >> >> > >
> >> >> > > 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
> >>
> >> About output section type (INFO):
> >> https://sourceware.org/binutils/docs/ld/Output-Section-Type.html#Output-Section-Type
> >> says "These type names are supported for backward compatibility, and are
> >> rarely used."
> >>
> >> If all input section don't have the SHF_ALLOC flag, the output section
> >> will not have this flag as well. This type is not useful...
> >>
> >> If .got and .got.plt were used, they should be considered dynamic
> >> relocations which should be part of the loadable image. So they should
> >> have the SHF_ALLOC flag. (INFO) will not be applicable anyway.
> >>
> >
> >I don't care deeply either way, but Kees indicated that he would like
> >to get rid of the 24 bytes of .got.plt magic entries that we have no
> >need for.
> >
> >In fact, a lot of this mangling is caused by the fact that the linker
> >is creating a relocatable binary, and assumes that it is a hosted
> >binary that is loaded by a dynamic loader. It would actually be much
> >better if the compiler and linker would take -ffreestanding into
> >account, and suppress GOT entries, PLTs, dynamic program headers for
> >shared libraries altogether.
>
> Linkers (GNU ld and LLD) don't create .got or .got.plt just because the linker
> command line has -pie or -shared.  They create .got or .got.plt if there are
> specific needs.
>
> For .got.plt, if there is (1) any .plt/.iplt entry, (2) any .got.plt based
> relocation (e.g. R_X86_64_GOTPC32 on x86-64), or (3) if _GLOBAL_OFFSET_TABLE_ is
> referenced, .got.plt will be created (both GNU ld and LLD) with usually 3
> entries (for ld.so purposes).
>

This is not the case for AArch64. There, __GLOBAL_OFFSET_TABLE__ is
always emitted, along with the magic .got.plt entries, regardless of
the input.

As for the input objects - why is '#pragma GCC visibility(hidden)' not
the default for -ffreestanding builds? This suppresses any GOT entries
emitted by the compiler, but the only way to get this behavior is
through the #pragma, which is how we ended up with '-include hidden.h'
in a couple of places.

IOW, if the toolchain behavior was not 100% geared towards shared
executables as it is today, we would not need the hacks that we need
to apply to get a relocatable bare metal binary like we need for the
KASLR kernel.


> If (1) is not satisfied, the created .got.plt is just served as an anchor for
> things that want to reference (the distance from GOT base to some point). The
> linker will still reserve 3 words but the words are likely not needed.
>
> I don't think there is a specific need for another option to teach the linker
> (GNU ld or LLD) that this is a kernel link.  For -ffreestanding builds, cc
> -static (ld -no-pie))/-static-pie (-pie) already work quite well.

You mean 'ld -static -pie' right? That seems to work. Is that a recent
invention?