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

[Jan Kiszka]

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@xxxxxxxxxxx> wrote:
>>>
>>> From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxxx>
>>> ---
>>>
>>> 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.

Jan

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