efi: zboot specific mechanism for embedding SBAT section

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). Put '.sbat' section in between '.data' and
'.text' as the former also covers '.bss' and thus must be the last one.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Author: vittyvk

drivers/firmware/efi/Kconfig

@@ -281,6 +281,30 @@ config EFI_EMBEDDED_FIRMWARE
 	bool
 	select CRYPTO_LIB_SHA256
 
+config EFI_SBAT
+       def_bool y if EFI_SBAT_FILE!=""
+
+config EFI_SBAT_FILE
+	string "Embedded SBAT section file path"
+	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.
+
+	  Specify a file with SBAT data which is going to be embedded as '.sbat'
+	  section into the kernel.
+
+	  If unsure, leave blank.
+
 endmenu
 
 config UEFI_CPER

drivers/firmware/efi/libstub/Makefile.zboot

@@ -44,6 +44,10 @@ 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)
 
+ifneq ($(CONFIG_EFI_SBAT_FILE),)
+$(obj)/zboot-header.o: $(CONFIG_EFI_SBAT_FILE)
+endif
+
 ZBOOT_DEPS := $(obj)/zboot-header.o $(objtree)/drivers/firmware/efi/libstub/lib.a
 
 LDFLAGS_vmlinuz.efi.elf := -T $(srctree)/drivers/firmware/efi/libstub/zboot.lds

drivers/firmware/efi/libstub/zboot-header.S

@@ -123,11 +123,29 @@ __efistub_efi_zboot_header:
 			IMAGE_SCN_MEM_READ | \
 			IMAGE_SCN_MEM_EXECUTE
 
+#ifdef CONFIG_EFI_SBAT
+	.ascii		".sbat\0\0\0"
+	.long		__sbat_size
+	.long		_sbat - .Ldoshdr
+	.long		__sbat_size
+	.long		_sbat - .Ldoshdr
+
+	.long		0, 0
+	.short		0, 0
+	.long		IMAGE_SCN_CNT_INITIALIZED_DATA | \
+			IMAGE_SCN_MEM_READ | \
+			IMAGE_SCN_MEM_DISCARDABLE
+
+	.pushsection ".sbat", "a", @progbits
+	.incbin CONFIG_EFI_SBAT_FILE
+	.popsection
+#endif
+
 	.ascii		".data\0\0\0"
 	.long		__data_size
-	.long		_etext - .Ldoshdr
+	.long		_data - .Ldoshdr
 	.long		__data_rawsize
-	.long		_etext - .Ldoshdr
+	.long		_data - .Ldoshdr
 
 	.long		0, 0
 	.short		0, 0

drivers/firmware/efi/libstub/zboot.lds

@@ -29,7 +29,17 @@ SECTIONS
 		. = _etext;
 	}
 
+#ifdef CONFIG_EFI_SBAT
+        .sbat : ALIGN(4096) {
+		_sbat = .;
+		*(.sbat)
+		_esbat = ALIGN(4096);
+		. = _esbat;
+	}
+#endif
+
 	.data : ALIGN(4096) {
+		_data = .;
 		*(.data* .init.data*)
 		_edata = ALIGN(512);
 		. = _edata;
@@ -52,3 +62,4 @@ PROVIDE(__efistub__gzdata_size =
 
 PROVIDE(__data_rawsize = ABSOLUTE(_edata - _etext));
 PROVIDE(__data_size = ABSOLUTE(_end - _etext));
+PROVIDE(__sbat_size = ABSOLUTE(_esbat - _sbat));