linux-kernel - Re: [PATCH 2/2] x86/efi: Implement support for embedding SBAT data for x86

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Message-ID: <CAMj1kXFMmhROmaDZ0gsw+ozG5iSkMvSXb15qexToUSAFyBn5hQ@mail.gmail.com>
Date: Fri, 25 Apr 2025 08:03:00 +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 2/2] x86/efi: Implement support for embedding SBAT data
 for x86

On Thu, 24 Apr 2025 at 10:10, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Similar to zboot architectures, implement support for embedding SBAT data
> for x86. Put '.sbat' section to the very end of the binary.
>
> Note, the obsolete CRC-32 checksum (see commit 9c54baab4401 ("x86/boot:
> Drop CRC-32 checksum and the build tool that generates it")) is gone and
> while it would've been possible to reserve the last 4 bytes in '.sbat'
> section too (like it's done today in '.data'), it seems to be a pointless
> exercise: SBAT makes zero sense without a signature on the EFI binary so
> '.sbat' won't be at the very end of the file anyway. Any tool which uses
> the last 4 bytes of the file as a checksum is broken with signed EFI
> binaries already.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  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           |  2 +-
>  5 files changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> index 81f55da81967..5f7b52f0e7f5 100644
> --- a/arch/x86/boot/Makefile
> +++ b/arch/x86/boot/Makefile
> @@ -71,7 +71,7 @@ $(obj)/vmlinux.bin: $(obj)/compressed/vmlinux FORCE
>
>  SETUP_OBJS = $(addprefix $(obj)/,$(setup-y))
>
> -sed-zoffset := -e 's/^\([0-9a-fA-F]*\) [a-zA-Z] \(startup_32\|efi.._stub_entry\|efi\(32\)\?_pe_entry\|input_data\|kernel_info\|_end\|_ehead\|_text\|_e\?data\|z_.*\)$$/\#define ZO_\2 0x\1/p'
> +sed-zoffset := -e 's/^\([0-9a-fA-F]*\) [a-zA-Z] \(startup_32\|efi.._stub_entry\|efi\(32\)\?_pe_entry\|input_data\|kernel_info\|_end\|_ehead\|_text\|_e\?data\|_e\?sbat\|z_.*\)$$/\#define ZO_\2 0x\1/p'
>
>  quiet_cmd_zoffset = ZOFFSET $@
>        cmd_zoffset = $(NM) $< | sed -n $(sed-zoffset) > $@
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index fdbce022db55..b9b80eccdc02 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -107,6 +107,8 @@ vmlinux-objs-$(CONFIG_UNACCEPTED_MEMORY) += $(obj)/mem.o
>  vmlinux-objs-$(CONFIG_EFI) += $(obj)/efi.o
>  vmlinux-libs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
>
> +vmlinux-objs-$(CONFIG_EFI_SBAT) += $(objtree)/drivers/firmware/efi/libstub/sbat.o
> +

Please drop this, and put the .incbin directly into header.S

>  $(obj)/vmlinux: $(vmlinux-objs-y) $(vmlinux-libs-y) FORCE
>         $(call if_changed,ld)
>
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 3b2bc61c9408..d0a27905de90 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -49,9 +49,22 @@ SECTIONS
>                 *(.data.*)
>
>                 /* Add 4 bytes of extra space for the obsolete CRC-32 checksum */
> +#ifndef CONFIG_EFI_SBAT
>                 . = ALIGN(. + 4, 0x200);
> +#else
> +               /* Avoid gap between '.data' and '.sbat' */
> +               . = ALIGN(. + 4, 0x1000);
> +#endif
>                 _edata = . ;
>         }
> +#ifdef CONFIG_EFI_SBAT
> +       .sbat : ALIGN(0x1000) {
> +               _sbat = . ;
> +               *(.sbat)
> +               _esbat = ALIGN(0x200);
> +               . = _esbat;
> +       }
> +#endif
>         . = ALIGN(L1_CACHE_BYTES);
>         .bss : {
>                 _bss = . ;

This looks a bit odd - see below

> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index b5c79f43359b..ab851490ef74 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -207,6 +207,19 @@ pecompat_fstart:
>                 IMAGE_SCN_MEM_READ              | \
>                 IMAGE_SCN_MEM_WRITE             # Characteristics
>
> +#ifdef CONFIG_EFI_SBAT
> +       .ascii ".sbat\0\0\0"
> +       .long   ZO__esbat - ZO__sbat            # VirtualSize
> +       .long   setup_size + ZO__sbat           # VirtualAddress
> +       .long   ZO__esbat - ZO__sbat            # SizeOfRawData
> +       .long   setup_size + ZO__sbat           # PointerToRawData
> +
> +       .long   0, 0, 0
> +       .long   IMAGE_SCN_CNT_INITIALIZED_DATA  | \
> +               IMAGE_SCN_MEM_READ              | \
> +               IMAGE_SCN_MEM_DISCARDABLE       # Characteristics
> +#endif
> +

This puts the .sbat section at the very end of the file. However, the
virtual size of .data is 'ZO__end - ZO__data' not 'ZO__edata -
ZO__data', and so the .sbat section will overlap with .bss in the
memory view of the image.


>         .set    section_count, (. - section_table) / 40
>  #endif /* CONFIG_EFI_STUB */
>
> diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
> index 2edb0167ba49..5022a378fec1 100644
> --- a/drivers/firmware/efi/Kconfig
> +++ b/drivers/firmware/efi/Kconfig
> @@ -283,7 +283,7 @@ config EFI_EMBEDDED_FIRMWARE
>
>  config EFI_SBAT
>         bool "Embed SBAT section in the kernel"
> -       depends on EFI_ZBOOT
> +       depends on EFI_ZBOOT || (EFI_STUB && X86)
>         help
>           SBAT section provides a way to improve SecureBoot revocations of UEFI
>           binaries by introducing a generation-based mechanism. With SBAT, older
> --
> 2.49.0
>
>