Re: [PATCH] riscv/efistub: Ensure GP-relative addressing is not used

[Jan Kiszka <jan.kiszka@siemens.com>]

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. :)

Jan

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