| Message ID | 20250424080950.289864-1-vkuznets@redhat.com |
|---|---|
| Headers | show |
| Series | efi: Add a mechanism for embedding SBAT section | expand |
Hi Vitaly, 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. > 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 > + $(call filechk,sbat.bin) > + Please get rid of all of this, and move the .incbin into zboot-header.S > # Even when -mbranch-protection=none is set, Clang will generate a > # .note.gnu.property for code-less object files (like lib/ctype.c), > # so work around this by explicitly removing the unwanted section. > diff --git a/drivers/firmware/efi/libstub/Makefile.zboot b/drivers/firmware/efi/libstub/Makefile.zboot > index 48842b5c106b..3d2d0b326f7c 100644 > --- a/drivers/firmware/efi/libstub/Makefile.zboot > +++ b/drivers/firmware/efi/libstub/Makefile.zboot > @@ -44,7 +44,8 @@ AFLAGS_zboot-header.o += -DMACHINE_TYPE=IMAGE_FILE_MACHINE_$(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. > +#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. > . = _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. > -- > 2.49.0 >
Changes since RFC: (https://lore.kernel.org/linux-efi/20250305101744.1706803-1-vkuznets@redhat.com/) - Implement SBAT embedding for zboot. (Smoke tested on aarch64 only, apologies if I missed some important differences on other arches!) It would've been possible to implement SBAT embedding for !zboot case too I think but this looks like an unnecessary complication: SecureBoot signing is likely to be done by distro vendors only and these will likely want zboot enabled anyway. - x86: Thanks to Ard, CRC32 is now gone (see commit 9c54baab4401 ("x86/boot: Drop CRC-32 checksum and the build tool that generates it")) and thus SBAT can be placed to the very end of the binary, this simplifies things a bit. 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 Currently, shim checks .sbat data for itself in self-test and for second stage bootloaders (grub, sd-boot, UKIs with sd-stub, ...) but kernel revocations require cycling signing keys or adding kernel hashes to shim's internal dbx. Adding .sbat to kernel and enforcing it on kernel loading will allow to do the same tracking and revocation distros are already doing with a simplified mechanism, and without having to keep lists of kernels outside of the git repos. Previously, an attempt was made to add ".sbat" section to the linux kernel: https://lwn.net/Articles/938422/ The approach was rejected mainly because currently there's no policy on how to update SBAT generation number when a new vulnerability is discovered. In particular, it is unclear what to do with stable kernels which may or may not backport certain patches making it impossible to describe the current state with a simple number. This series suggests a different approach: instead of defining SBAT information, provide a mechanism for downstream kernel builders (distros) to include their own SBAT data. This leaves the decision on the policy to the distro vendors. 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'. Linux upstream will never, ever need to care about the data unless they choose in the future to participate in that way. Vitaly Kuznetsov (2): efi/libstub: zboot specific mechanism for embedding SBAT section x86/efi: Implement support for embedding SBAT data for x86 arch/x86/boot/Makefile | 2 +- arch/x86/boot/compressed/Makefile | 2 ++ arch/x86/boot/compressed/vmlinux.lds.S | 13 +++++++++++ arch/x86/boot/header.S | 13 +++++++++++ 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 ++++++++++++++ 10 files changed, 101 insertions(+), 2 deletions(-) create mode 100644 drivers/firmware/efi/libstub/sbat.S