Re: [RFC PATCH v2] x86/boot: add .sbat section to the bzImage

[H. Peter Anvin]

On July 11, 2023 8:44:49 AM PDT, Emanuele Giuseppe Esposito <eesposit@xxxxxxxxxx> wrote:
>*Important*: this is just an RFC, as I am not expert in this area and
>I don't know what's the best way to achieve this.
>
>v2:
>* add standard "sbat,1,SBAT Version,..." header string
>
>The aim of this patch is to add a .sbat section to the linux binary
>(https://github.com/rhboot/shim/blob/main/SBAT.md).
>We mainly need SBAT in UKIs (Unified Kernel Images), as we might want
>to revoke authorizations to specific signed PEs that were initially
>considered as trusted. The reason might be for example a security issue
>related to a specific linux release.
>
>A .sbat is simply a section containing a string with the component name
>and a version number. This version number is compared with the value in
>OVMF_VARS, and if it's less than the variable, the binary is not trusted,
>even if it is correctly signed.
>
>Right now an UKI is built with a .sbat section containing the
>systemd-stub sbat string (upstream + vendor), we would like to add
>also a per-component specific string (ie vmlinux has its own sbat,
>again upstream + vendor, each signed add-on its own and so on).
>In this way, if a specific kernel version has an issue, we can revoke
>it without compromising all other UKIs that are using a different
>kernel with the same stub/initrd/something else.
>
>Issues with this patch:
>* the string is added in a file but it is never deleted
>* if the code is not modified but make is issued again, objcopy will
>  be called again and will fail because .sbat exists already, making
>  compilation fail
>* minor display issue: objcopy command is printed in the make logs
>
>Signed-off-by: Emanuele Giuseppe Esposito <eesposit@xxxxxxxxxx>
>---
> arch/x86/boot/Makefile | 3 +++
> 1 file changed, 3 insertions(+)
>
>diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
>index 9e38ffaadb5d..6982a50ba0c0 100644
>--- a/arch/x86/boot/Makefile
>+++ b/arch/x86/boot/Makefile
>@@ -83,6 +83,9 @@ cmd_image = $(obj)/tools/build $(obj)/setup.bin $(obj)/vmlinux.bin \
> 
> $(obj)/bzImage: $(obj)/setup.bin $(obj)/vmlinux.bin $(obj)/tools/build FORCE
> 	$(call if_changed,image)
>+	@$(kecho) "sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md"; > linux.sbat
>+	@$(kecho) "linux,1,The Linux Developers,linux,$(KERNELVERSION),https://linux.org"; >> linux.sbat;
>+	$(OBJCOPY) --set-section-alignment '.sbat=512' --add-section .sbat=linux.sbat $@;
> 	@$(kecho) 'Kernel: $@ is ready' ' (#'$(or $(KBUILD_BUILD_VERSION),`cat .version`)')'
> 
> OBJCOPYFLAGS_vmlinux.bin := -O binary -R .note -R .comment -S

Why is this a special section rather than in the kernel_info structure?