linux-kernel - Re: [PATCH v2 1/2] efi: zboot specific mechanism for embedding SBAT section

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Message-ID: <CAMj1kXHSW1=k59J9vOQULw2A_htLiKYc0h9zQ12xYqmutja+eg@mail.gmail.com>
Date: Fri, 9 May 2025 11:16:44 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: x86@...nel.org, linux-efi@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Peter Jones <pjones@...hat.com>, Daniel Berrange <berrange@...hat.com>, 
	Emanuele Giuseppe Esposito <eesposit@...hat.com>, Gerd Hoffmann <kraxel@...hat.com>, 
	Greg KH <gregkh@...uxfoundation.org>, Luca Boccassi <bluca@...ian.org>, 
	Peter Zijlstra <peterz@...radead.org>, Matthew Garrett <mjg59@...f.ucam.org>, 
	James Bottomley <James.Bottomley@...senpartnership.com>, 
	Eric Snowberg <eric.snowberg@...cle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>, linux-riscv@...ts.infradead.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] efi: zboot specific mechanism for embedding SBAT section

On Mon, 5 May 2025 at 17:45, Vitaly Kuznetsov <vkuznets@...hat.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). Put '.sbat' section in between '.data' and
> '.text' as the former also covers '.bss' and thus must be the last one.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>

Reviewed-by: Ard Biesheuvel <ardb@...nel.org>

> ---
>  drivers/firmware/efi/Kconfig                | 24 +++++++++++++++++++++
>  drivers/firmware/efi/libstub/Makefile.zboot |  4 ++++
>  drivers/firmware/efi/libstub/zboot-header.S | 22 +++++++++++++++++--
>  drivers/firmware/efi/libstub/zboot.lds      | 11 ++++++++++
>  4 files changed, 59 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
> index 5fe61b9ab5f9..db8c5c03d3a2 100644
> --- a/drivers/firmware/efi/Kconfig
> +++ b/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
> diff --git a/drivers/firmware/efi/libstub/Makefile.zboot b/drivers/firmware/efi/libstub/Makefile.zboot
> index 48842b5c106b..92e3c73502ba 100644
> --- a/drivers/firmware/efi/libstub/Makefile.zboot
> +++ b/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
> diff --git a/drivers/firmware/efi/libstub/zboot-header.S b/drivers/firmware/efi/libstub/zboot-header.S
> index fb676ded47fa..e02247458b65 100644
> --- a/drivers/firmware/efi/libstub/zboot-header.S
> +++ b/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
> diff --git a/drivers/firmware/efi/libstub/zboot.lds b/drivers/firmware/efi/libstub/zboot.lds
> index 9ecc57ff5b45..c3a166675450 100644
> --- a/drivers/firmware/efi/libstub/zboot.lds
> +++ b/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));
> --
> 2.49.0
>