[v2,08/14] arm64: efi: split Image code and data into separate PE/COFF sections

To prevent unintended modifications to the kernel text (malicious or
otherwise) while running the EFI stub, describe the kernel image as
two separate sections: a .text section with read-execute permissions,
covering .text, .rodata and .init.text, and a .data section with
read-write permissions, covering .init.data, .data and .bss.

This relies on the firmware to actually take the section permission
flags into account, but this is something that is currently being
implemented in EDK2, which means we will likely start seeing it in
the wild between one and two years from now.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

---
 arch/arm64/kernel/efi-header.S  | 23 +++++++++++++++-----
 arch/arm64/kernel/vmlinux.lds.S |  5 +++++
 2 files changed, 23 insertions(+), 5 deletions(-)

-- 
2.7.4

--
To unsubscribe from this list: send the line "unsubscribe linux-efi" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

On Wed, Feb 08, 2017 at 11:55:41AM +0000, Ard Biesheuvel wrote:
> To prevent unintended modifications to the kernel text (malicious or

> otherwise) while running the EFI stub, describe the kernel image as

> two separate sections: a .text section with read-execute permissions,

> covering .text, .rodata and .init.text, and a .data section with

> read-write permissions, covering .init.data, .data and .bss.

> 

> This relies on the firmware to actually take the section permission

> flags into account, but this is something that is currently being

> implemented in EDK2, which means we will likely start seeing it in

> the wild between one and two years from now.

> 

> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>


> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S

> index b8deffa9e1bf..a93cc2b6f50b 100644

> --- a/arch/arm64/kernel/vmlinux.lds.S

> +++ b/arch/arm64/kernel/vmlinux.lds.S

> @@ -149,6 +149,9 @@ SECTIONS

>  		ARM_EXIT_KEEP(EXIT_TEXT)

>  	}

>  

> +	. = ALIGN(SZ_4K);

> +	__pecoff_data_start = .;

> +


I understand that the stub needs to split the init text/data since
unlike the kernel it'll map those with separate permissions, but it
feels odd to do this specifically for the EFI stub.

Yould it perhaps make more sense to always use separate segments for
init/exit text/data, and also apply the permission split in the kernel?

With that, I don't think we'd need additional stub-specific linker
script changes.

Thanks,
Mark.

>  	.init.data : {

>  		INIT_DATA

>  		INIT_SETUP(16)

> @@ -206,6 +209,7 @@ SECTIONS

>  	}

>  

>  	PECOFF_EDATA_PADDING

> +	__pecoff_data_rawsize = ABSOLUTE(. - __pecoff_data_start);

>  	_edata = .;

>  

>  	BSS_SECTION(0, 0, 0)

> @@ -221,6 +225,7 @@ SECTIONS

>  	. += RESERVED_TTBR0_SIZE;

>  #endif

>  

> +	__pecoff_data_size = ABSOLUTE(. - __pecoff_data_start);

>  	_end = .;

>  

>  	STABS_DEBUG

> -- 

> 2.7.4

> 

--
To unsubscribe from this list: send the line "unsubscribe linux-efi" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

On 10 February 2017 at 10:49, Mark Rutland <mark.rutland@arm.com> wrote:
> On Wed, Feb 08, 2017 at 11:55:41AM +0000, Ard Biesheuvel wrote:

>> To prevent unintended modifications to the kernel text (malicious or

>> otherwise) while running the EFI stub, describe the kernel image as

>> two separate sections: a .text section with read-execute permissions,

>> covering .text, .rodata and .init.text, and a .data section with

>> read-write permissions, covering .init.data, .data and .bss.

>>

>> This relies on the firmware to actually take the section permission

>> flags into account, but this is something that is currently being

>> implemented in EDK2, which means we will likely start seeing it in

>> the wild between one and two years from now.

>>

>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

>

>> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S

>> index b8deffa9e1bf..a93cc2b6f50b 100644

>> --- a/arch/arm64/kernel/vmlinux.lds.S

>> +++ b/arch/arm64/kernel/vmlinux.lds.S

>> @@ -149,6 +149,9 @@ SECTIONS

>>               ARM_EXIT_KEEP(EXIT_TEXT)

>>       }

>>

>> +     . = ALIGN(SZ_4K);

>> +     __pecoff_data_start = .;

>> +

>

> I understand that the stub needs to split the init text/data since

> unlike the kernel it'll map those with separate permissions, but it

> feels odd to do this specifically for the EFI stub.

>


While the init code executes in a *much* more controlled environment
than the stub (which invokes various UEFI boot services to load
initrds/dtb from block storage, and may do god knows what during
ExitBootServices()), I think it is not unreasonable to split the init
mapping into rx/rw segments, given that it is the only place where we
have a good chunk of memory that is both writable and executable.

> Yould it perhaps make more sense to always use separate segments for

> init/exit text/data, and also apply the permission split in the kernel?

>

> With that, I don't think we'd need additional stub-specific linker

> script changes.

>


I will prototype this
--
To unsubscribe from this list: send the line "unsubscribe linux-efi" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html