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Date:   Tue, 11 Jul 2023 18:21:24 -0700
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Emanuele Giuseppe Esposito <eesposit@redhat.com>, x86@kernel.org
CC:     Thomas Gleixner <tglx@linutronix.de>, bluca@debian.org,
        lennart@poettering.net, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        =?ISO-8859-1?Q?Daniel_P_=2E_Berrang=E9?= <berrange@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2] x86/boot: add .sbat section to the bzImage
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On July 11, 2023 8:44:49 AM PDT, Emanuele Giuseppe Esposito <eesposit@redha=
t=2Ecom> 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=2E
>
>v2:
>* add standard "sbat,1,SBAT Version,=2E=2E=2E" header string
>
>The aim of this patch is to add a =2Esbat section to the linux binary
>(https://github=2Ecom/rhboot/shim/blob/main/SBAT=2Emd)=2E
>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=2E The reason might be for example a security issue
>related to a specific linux release=2E
>
>A =2Esbat is simply a section containing a string with the component name
>and a version number=2E 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=2E
>
>Right now an UKI is built with a =2Esbat 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)=2E
>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=2E
>
>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 =2Esbat exists already, making
>  compilation fail
>* minor display issue: objcopy command is printed in the make logs
>
>Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat=2Ecom>
>---
> arch/x86/boot/Makefile | 3 +++
> 1 file changed, 3 insertions(+)
>
>diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
>index 9e38ffaadb5d=2E=2E6982a50ba0c0 100644
>--- a/arch/x86/boot/Makefile
>+++ b/arch/x86/boot/Makefile
>@@ -83,6 +83,9 @@ cmd_image =3D $(obj)/tools/build $(obj)/setup=2Ebin $(o=
bj)/vmlinux=2Ebin \
>=20
> $(obj)/bzImage: $(obj)/setup=2Ebin $(obj)/vmlinux=2Ebin $(obj)/tools/bui=
ld FORCE
> 	$(call if_changed,image)
>+	@$(kecho) "sbat,1,SBAT Version,sbat,1,https://github=2Ecom/rhboot/shim/=
blob/main/SBAT=2Emd" > linux=2Esbat
>+	@$(kecho) "linux,1,The Linux Developers,linux,$(KERNELVERSION),https://=
linux=2Eorg" >> linux=2Esbat;
>+	$(OBJCOPY) --set-section-alignment '=2Esbat=3D512' --add-section =2Esba=
t=3Dlinux=2Esbat $@;
> 	@$(kecho) 'Kernel: $@ is ready' ' (#'$(or $(KBUILD_BUILD_VERSION),`cat =
=2Eversion`)')'
>=20
> OBJCOPYFLAGS_vmlinux=2Ebin :=3D -O binary -R =2Enote -R =2Ecomment -S

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