Re: [PATCH 1/2] efi/libstub: zboot specific mechanism for embedding SBAT section

Vitaly Kuznetsov
2025-04-28T11:00:43Z

Ard Biesheuvel <ardb@kernel.org> writes:

> Hi Vitaly,
>

Ard, thanks for the review!

> On Thu, 24 Apr 2025 at 10:10, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>>
>> SBAT is a mechanism which improves SecureBoot revocations of UEFI binaries
>> by introducing a generation-based technique. Compromised or vulnerable UEFI
>> binaries can be prevented from booting by bumping the minimal required
>> generation for the specific component in the bootloader. More information
>> on the SBAT can be obtained here:
>>
>> https://github.com/rhboot/shim/blob/main/SBAT.md
>>
>> Upstream Linux kernel does not currently participate in any way in SBAT as
>> there's no existing policy in how SBAT generation number should be
>> defined. Keep the status quo and provide a mechanism for distro vendors and
>> anyone else who signs their kernel for SecureBoot to include their own SBAT
>> data. This leaves the decision on the policy to the vendor. Basically, each
>> distro implementing SecureBoot today, will have an option to inject their
>> own SBAT data during kernel build and before it gets signed by their
>> SecureBoot CA. Different distro do not need to agree on the common SBAT
>> component names or generation numbers as each distro ships its own 'shim'
>> with their own 'vendor_cert'/'vendor_db'
>>
>> Implement support for embedding SBAT data for architectures using
>> zboot (arm64, loongarch, riscv). Build '.sbat' section along with libstub
>> so it can be reused by x86 implementation later.
>>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>> drivers/firmware/efi/Kconfig | 25 +++++++++++++++++++++
>> drivers/firmware/efi/libstub/Makefile | 7 ++++++
>> drivers/firmware/efi/libstub/Makefile.zboot | 3 ++-
>> drivers/firmware/efi/libstub/sbat.S | 7 ++++++
>> drivers/firmware/efi/libstub/zboot-header.S | 14 ++++++++++++
>> drivers/firmware/efi/libstub/zboot.lds | 17 ++++++++++++++
>> 6 files changed, 72 insertions(+), 1 deletion(-)
>> create mode 100644 drivers/firmware/efi/libstub/sbat.S
>>
>> diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
>> index 5fe61b9ab5f9..2edb0167ba49 100644
>> --- a/drivers/firmware/efi/Kconfig
>> +++ b/drivers/firmware/efi/Kconfig
>> @@ -281,6 +281,31 @@ config EFI_EMBEDDED_FIRMWARE
>> bool
>> select CRYPTO_LIB_SHA256
>>
>> +config EFI_SBAT
>> + bool "Embed SBAT section in the kernel"
>> + depends on EFI_ZBOOT
>> + help
>> + SBAT section provides a way to improve SecureBoot revocations of UEFI
>> + binaries by introducing a generation-based mechanism. With SBAT, older
>> + UEFI binaries can be prevented from booting by bumping the minimal
>> + required generation for the specific component in the bootloader.
>> +
>> + Note: SBAT information is distribution specific, i.e. the owner of the
>> + signing SecureBoot certificate must define the SBAT policy. Linux
>> + kernel upstream does not define SBAT components and their generations.
>> +
>> + See https://github.com/rhboot/shim/blob/main/SBAT.md for the additional
>> + details.
>> +
>> + If unsure, say N.
>> +
>> +config EFI_SBAT_FILE
>> + string "Embedded SBAT section file path"
>> + depends on EFI_SBAT
>> + help
>> + Specify a file with SBAT data which is going to be embedded as '.sbat'
>> + section into the kernel.
>> +
>
> Can we simplify this? CONFIG_EFI_SBAT makes no sense if
> CONFIG_EFI_SBAT_FILE is left empty. If you really need both symbols,
> set EFI_SBAT automatically based on whether EFI_SBAT_FILE is
> non-empty.

Sure, but FWIW, I modelled this after MODULE_SIG/MODULE_SIG_KEY and
BOOT_CONFIG_EMBED/BOOT_CONFIG_EMBED_FILE where the selection is also
2-step -- do you think EFI_SBAT/EFI_SBAT_FILE case is different?

>
>> endmenu
>>
>> config UEFI_CPER
>> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
>> index d23a1b9fed75..5113cbdadf9a 100644
>> --- a/drivers/firmware/efi/libstub/Makefile
>> +++ b/drivers/firmware/efi/libstub/Makefile
>> @@ -105,6 +105,13 @@ lib-$(CONFIG_UNACCEPTED_MEMORY) += unaccepted_memory.o bitmap.o find.o
>> extra-y := $(lib-y)
>> lib-y := $(patsubst %.o,%.stub.o,$(lib-y))
>>
>> +extra-$(CONFIG_EFI_SBAT) += sbat.o
>> +$(obj)/sbat.o: $(obj)/sbat.bin
>> +targets += sbat.bin
>> +filechk_sbat.bin = cat $(or $(real-prereqs), /dev/null)
>> +$(obj)/sbat.bin: $(CONFIG_EFI_SBAT_FILE) FORCE
>> + $(call filechk,sbat.bin)
>> +
>
> Please get rid of all of this, and move the .incbin into
> zboot-header.S

The main prupose of this logic is to track possible sbat data
changes. E.g. if the file with SBAT data has changed, then we need to
rebuild the kernel binary. If we just use a raw 'incbin' somewhere and
don't add a specific Makefile dependency, then the logic will be lost.

I think I can drop the dedicated 'sbat.S' and use zboot-header.S but I'd
like to keep at least the 'filechk' part: we compare what's in
EFI_SBAT_FILE with 'sbat.bin' copy and, if things have changed, rebuild.

>
>
>> # Even when -mbranch-protection=none is set, Clang will generate a
>> # .note.gnu.property for code-less object files (like lib/ctype.c),
>> # so work around this by explicitly removing the unwanted section.
>> diff --git a/drivers/firmware/efi/libstub/Makefile.zboot b/drivers/firmware/efi/libstub/Makefile.zboot
>> index 48842b5c106b..3d2d0b326f7c 100644
>> --- a/drivers/firmware/efi/libstub/Makefile.zboot
>> +++ b/drivers/firmware/efi/libstub/Makefile.zboot
>> @@ -44,7 +44,8 @@ AFLAGS_zboot-header.o += -DMACHINE_TYPE=IMAGE_FILE_MACHINE_$(EFI_ZBOOT_MACH_TYPE
>> $(obj)/zboot-header.o: $(srctree)/drivers/firmware/efi/libstub/zboot-header.S FORCE
>> $(call if_changed_rule,as_o_S)
>>
>> -ZBOOT_DEPS := $(obj)/zboot-header.o $(objtree)/drivers/firmware/efi/libstub/lib.a
>> +ZBOOT_DEPS := $(obj)/zboot-header.o $(objtree)/drivers/firmware/efi/libstub/lib.a \
>> + $(if $(CONFIG_EFI_SBAT),$(objtree)/drivers/firmware/efi/libstub/sbat.o)
>>
>
> Drop this too
>
>> LDFLAGS_vmlinuz.efi.elf := -T $(srctree)/drivers/firmware/efi/libstub/zboot.lds
>> $(obj)/vmlinuz.efi.elf: $(obj)/vmlinuz.o $(ZBOOT_DEPS) FORCE
>> diff --git a/drivers/firmware/efi/libstub/sbat.S b/drivers/firmware/efi/libstub/sbat.S
>> new file mode 100644
>> index 000000000000..4e99a1bac794
>> --- /dev/null
>> +++ b/drivers/firmware/efi/libstub/sbat.S
>> @@ -0,0 +1,7 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/*
>> + * Embed SBAT data in the kernel.
>> + */
>> + .pushsection ".sbat","a",@progbits
>> + .incbin "drivers/firmware/efi/libstub/sbat.bin"
>> + .popsection
>> diff --git a/drivers/firmware/efi/libstub/zboot-header.S b/drivers/firmware/efi/libstub/zboot-header.S
>> index fb676ded47fa..f2df24504fc5 100644
>> --- a/drivers/firmware/efi/libstub/zboot-header.S
>> +++ b/drivers/firmware/efi/libstub/zboot-header.S
>> @@ -135,6 +135,20 @@ __efistub_efi_zboot_header:
>> IMAGE_SCN_MEM_READ | \
>> IMAGE_SCN_MEM_WRITE
>>
>> +#ifdef CONFIG_EFI_SBAT
>> + .ascii ".sbat\0\0\0"
>> + .long __sbat_size
>> + .long _edata - .Ldoshdr
>> + .long __sbat_size
>> + .long _edata - .Ldoshdr
>> +
>> + .long 0, 0
>> + .short 0, 0
>> + .long IMAGE_SCN_CNT_INITIALIZED_DATA | \
>> + IMAGE_SCN_MEM_READ | \
>> + IMAGE_SCN_MEM_DISCARDABLE
>
> You can put the pushsection/popsection right here.
>

Ok (but see my comment about Makefile magic above.

>> +#endif
>> +
>> .set .Lsection_count, (. - .Lsection_table) / 40
>>
>> #ifdef PE_DLL_CHAR_EX
>> diff --git a/drivers/firmware/efi/libstub/zboot.lds b/drivers/firmware/efi/libstub/zboot.lds
>> index 9ecc57ff5b45..2cd5015c70ce 100644
>> --- a/drivers/firmware/efi/libstub/zboot.lds
>> +++ b/drivers/firmware/efi/libstub/zboot.lds
>> @@ -31,10 +31,24 @@ SECTIONS
>>
>> .data : ALIGN(4096) {
>> *(.data* .init.data*)
>> +#ifndef CONFIG_EFI_SBAT
>> _edata = ALIGN(512);
>> +#else
>> + /* Avoid gap between '.data' and '.sbat' */
>> + _edata = ALIGN(4096);
>> +#endif
>
> Just use 4096 in all cases.
>

Ok.

>> . = _edata;
>> }
>>
>> +#ifdef CONFIG_EFI_SBAT
>> + .sbat : ALIGN(4096) {
>> + _sbat = . ;
>> + *(.sbat)
>> + _esbat = ALIGN(512);
>> + . = _esbat;
>> + }
>> +#endif
>> +
>> .bss : {
>> *(.bss* .init.bss*)
>> _end = ALIGN(512);
>> @@ -52,3 +66,6 @@ PROVIDE(__efistub__gzdata_size =
>>
>> PROVIDE(__data_rawsize = ABSOLUTE(_edata - _etext));
>> PROVIDE(__data_size = ABSOLUTE(_end - _etext));
>> +#ifdef CONFIG_EFI_SBAT
>> +PROVIDE(__sbat_size = ABSOLUTE(_esbat - _sbat));
>> +#endif
>
> This can be unconditional - it is only evaluated when a reference to it exists.
>

Yes, it compiles well, I put #ifdef here mostly for documentational
purposes. Will drop.

>> --
>> 2.49.0
>>
>

--
Vitaly