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

Vitaly Kuznetsov
2025-04-24T08:12:16Z

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.
+
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
+ $(call filechk,sbat.bin)
+
# Even when -mbranch-protection=none is set, Clang will generate a
# .note.gnu.property for code-less object files (like lib/ctype.c),
# so work around this by explicitly removing the unwanted section.
diff --git a/drivers/firmware/efi/libstub/Makefile.zboot b/drivers/firmware/efi/libstub/Makefile.zboot
index 48842b5c106b..3d2d0b326f7c 100644
--- a/drivers/firmware/efi/libstub/Makefile.zboot
+++ b/drivers/firmware/efi/libstub/Makefile.zboot
@@ -44,7 +44,8 @@ AFLAGS_zboot-header.o += -DMACHINE_TYPE=IMAGE_FILE_MACHINE_$(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)

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
+#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
. = _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
--
2.49.0