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From: Ard Biesheuvel <ardb@kernel.org>
Date: Tue, 16 Jan 2024 14:58:45 +0100
Message-ID: <CAMj1kXEb5+g1gLqnQsqWpkYFnDmyB1n2pwAUzW7bx8LqLE1WRA@mail.gmail.com>
Subject: Re: [PATCH] riscv/efistub: Ensure GP-relative addressing is not used
To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Palmer Dabbelt <palmer@rivosinc.com>, linux-efi@vger.kernel.org, 
	linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"

On Tue, 16 Jan 2024 at 14:56, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>
> On 16.01.24 14:47, Ard Biesheuvel wrote:
> > On Tue, 16 Jan 2024 at 14:44, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> >>
> >> On 16.01.24 09:36, Ard Biesheuvel wrote:
> >>> On Tue, 16 Jan 2024 at 06:21, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> >>>>
> >>>> On 15.01.24 18:34, Ard Biesheuvel wrote:
> >>>>> On Sat, 13 Jan 2024 at 11:35, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> >>>>>>
> >>>>>> On 12.01.24 19:56, Palmer Dabbelt wrote:
> >>>>>>> On Fri, 12 Jan 2024 10:51:16 PST (-0800), Ard Biesheuvel wrote:
> >>>>>>>> Hi Jan,
> >>>>>>>>
> >>>>>>>> On Fri, 12 Jan 2024 at 19:37, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> >>>>>>>>>
> >>>>>>>>> From: Jan Kiszka <jan.kiszka@siemens.com>
> >>>>>>>>>
> >>>>>>>>> The cflags for the RISC-V efistub were missing -mno-relax, thus were
> >>>>>>>>> under the risk that the compiler could use GP-relative addressing. That
> >>>>>>>>> happened for _edata with binutils-2.41 and kernel 6.1, causing the
> >>>>>>>>> relocation to fail due to an invalid kernel_size in handle_kernel_image.
> >>>>>>>>> It was not yet observed with newer versions, but that may just be luck.
> >>>>>>>>>
> >>>>>>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> >>>>>>>>> ---
> >>>>>>>>>
> >>>>>>>>> Something like this should go to stable as well, but we will need
> >>>>>>>>> rebased patches.
> >>>>>>>>>
> >>>>>>>>>  drivers/firmware/efi/libstub/Makefile | 2 +-
> >>>>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>>>>>>
> >>>>>>>>> diff --git a/drivers/firmware/efi/libstub/Makefile
> >>>>>>>>> b/drivers/firmware/efi/libstub/Makefile
> >>>>>>>>> index 06964a3c130f..d561d7de46a9 100644
> >>>>>>>>> --- a/drivers/firmware/efi/libstub/Makefile
> >>>>>>>>> +++ b/drivers/firmware/efi/libstub/Makefile
> >>>>>>>>> @@ -28,7 +28,7 @@ cflags-$(CONFIG_ARM)          += -DEFI_HAVE_STRLEN
> >>>>>>>>> -DEFI_HAVE_STRNLEN \
> >>>>>>>>>                                    -DEFI_HAVE_MEMCHR
> >>>>>>>>> -DEFI_HAVE_STRRCHR \
> >>>>>>>>>                                    -DEFI_HAVE_STRCMP -fno-builtin
> >>>>>>>>> -fpic \
> >>>>>>>>>                                    $(call
> >>>>>>>>> cc-option,-mno-single-pic-base)
> >>>>>>>>> -cflags-$(CONFIG_RISCV)         += -fpic -DNO_ALTERNATIVE
> >>>>>>>>> +cflags-$(CONFIG_RISCV)         += -fpic -DNO_ALTERNATIVE -mno-relax
> >>>>>>>>
> >>>>>>>> Can we detect the presence of these references (via the relocation
> >>>>>>>> type)? We already do something similar for ordinary absolute
> >>>>>>>> references too.
> >>>>>>>
> >>>>>>> If there's no `__global_pointer$` symbol then the linker won't make
> >>>>>>> GP-relative relaxations (because it doesn't know where GP is).  We
> >>>>>>> usually define that symbol in the linker script, but I'm not entierly
> >>>>>>> sure how libstub gets its linker script...
> >>>>>>>
> >>>>>>
> >>>>>> The stub seems to be linked together with the rest of the kernel, thus
> >>>>>> the regular arch/riscv/kernel/vmlinux.lds.S is used.
> >>>>>>
> >>>>>
> >>>>> Indeed - the EFI stub is part of the same executable as vmlinux, we
> >>>>> just mangle the symbol names to ensure that only code that can be
> >>>>> safely called from the EFI stub can be linked to it.
> >>>>>
> >>>>> If the effect of -mno-relax is to stop emitting R_RISCV_RELAX
> >>>>> relocations, we should perhaps add those to the STUBCOPY_RELOC-y
> >>>>> Makefile variable? (in the same file). BTW R_RISCV_HI20 doesn't seem
> >>>>> like the right value there to begin with: the idea of that is to
> >>>>> disallow ELF relocations that evaluate to expressions that can only be
> >>>>> known at runtime (like absolute addresses for global pointer
> >>>>> variables)
> >>>>
> >>>> How to do that best? Simply replace R_RISCV_HI20 with R_RISCV_RELAX?
> >>>>
> >>>
> >>> We'll need to keep the HI20, in fact - I got confused between HI20 and
> >>> PCREL_HI20, and the former is actually used for 32-bit absolute
> >>> addresses in 32-bit code.
> >>>
> >>> This seems to do the trick: it disallows relaxation relocations and
> >>> native word sizes absolute references. AFAICT, those are the only ones
> >>> we should care about.
> >>>
> >>> STUBCOPY_RELOC-$(CONFIG_RISCV) := -E
> >>> R_RISCV_HI20\|R_RISCV_$(BITS)\|R_RISCV_RELAX
> >>
> >> I would suggest to do that on top of this patch. Want me to write such a
> >> patch, or will you? You can probably more fluently explain why
> >> R_RISCV_32/64 is important, I would first have to understand what that
> >> is exactly. :)
> >>
> >
> > Sure, I can take care of that.
> >
>
> Perfect.
>
> > For your patch,
> >
> > Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
> >
> > I'll queue this up as a EFI fix.
>
> Thanks. Will you take care of stable, or should I once the commit was
> merged?
>

I'll add the cc:stable so we'll both get informed once the -stable
maintainers [attempt to] queue this up.