Re: [RFC v2 PATCH] x86/boot: Add the secdata section to the setup header

From: Gary Lin
Date: Thu Jun 01 2017 - 04:12:24 EST

On Fri, May 12, 2017 at 04:05:34PM +0800, Gary Lin wrote:
> A new section, secdata, in the setup header is introduced to store the
> distro-specific security version which is designed to help the
> bootloader to warn the user when loading a less secure or vulnerable
> kernel. The secdata section can be presented as the following:
>
> struct sec_hdr {
> __u16 header_length;
> __u32 distro_version;
> __u16 security_version;
> } __attribute__((packed));
> char *signer;
>
> It consists of a fixed size structure and a null-terminated string.
> "header_length" is the size of "struct sec_hdr" and can be used as the
> offset to "signer". It also can be a kind of the "header version" to
> detect if any new member is introduced.
>
> The kernel packager of the distribution can put the distro name in
> "signer" and the distro version in "distro_version". When a severe
> vulnerability is fixed, the packager increases "security_version" in
> the kernel build afterward. The bootloader can maintain a list of the
> security versions of the current kernels and only allows the kernel with
> a higher or equal security version to boot. If the user is going to boot
> a kernel with a lower security version, a warning should show to prevent
> the user from loading a vulnerable kernel accidentally.
>
> Enabling UEFI Secure Boot is recommended when using the security version
> or the attacker may alter the security version stealthily.
>
Any comment?

Gary Lin

> v2:
> - Decrease the size of secdata_offset to 2 bytes since the setup header
> is limited to around 32KB.
> - Restructure the secdata section. The signer is now a null-terminated
> string. The type of distro_version changes to u32 in case the distro
> uses a long version.
> - Modify the Kconfig names and add help.
> - Remove the signer name hack in build.c.
>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Joey Lee <jlee@xxxxxxxx>
> Signed-off-by: Gary Lin <glin@xxxxxxxx>
> ---
> arch/x86/Kconfig | 28 ++++++++++++++++++++++++++++
> arch/x86/boot/header.S | 14 +++++++++++++-
> arch/x86/boot/setup.ld | 1 +
> arch/x86/boot/tools/build.c | 1 -
> arch/x86/include/uapi/asm/bootparam.h | 1 +
> 5 files changed, 43 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 5bbdef151805..2c5539518ce0 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1817,6 +1817,34 @@ config EFI_MIXED
>
> If unsure, say N.
>
> +config SIGNER_NAME
> + string "Signer name"
> + default ""
> + ---help---
> + This option specifies who signs or releases this kernel.
> +
> +config DISTRO_VERSION
> + int "Distribution version"
> + default 0
> + range 0 4294967295
> + ---help---
> + This option specifies the distribution version which this
> + kernel belongs to.
> +
> +config SECURITY_VERSION
> + int "Security version"
> + default 0
> + range 0 65535
> + ---help---
> + The security version is the version defined by the distribution
> + to indicate the severe security fixes. The bootloader can maintain
> + a list of the security versions of the current kernels. After
> + fixing a severe vulnerability in the kernel, the distribution can
> + increase the security version to notify the bootloader to update
> + the list. When booting a kernel with a lower security version,
> + the bootloader warns the user to avoid loading a vulnerable kernel
> + accidentally.
> +
> config SECCOMP
> def_bool y
> prompt "Enable seccomp to safely compute untrusted bytecode"
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 3dd5be33aaa7..37683caf1668 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -301,7 +301,7 @@ _start:
> # Part 2 of the header, from the old setup.S
>
> .ascii "HdrS" # header signature
> - .word 0x020d # header version number (>= 0x0105)
> + .word 0x020e # header version number (>= 0x0105)
> # or else old loadlin-1.5 will fail)
> .globl realmode_swtch
> realmode_swtch: .word 0, 0 # default_switch, SETUPSEG
> @@ -552,6 +552,7 @@ pref_address: .quad LOAD_PHYSICAL_ADDR # preferred load addr
>
> init_size: .long INIT_SIZE # kernel initialization size
> handover_offset: .long 0 # Filled in by build.c
> +secdata_offset: .word secdata_start
>
> # End of setup header #####################################################
>
> @@ -629,3 +630,14 @@ die:
> setup_corrupt:
> .byte 7
> .string "No setup signature found...\n"
> +
> + .section ".secdata", "a"
> +secdata_start:
> +header_length:
> + .word signer - secdata_start
> +distro_version:
> + .long CONFIG_DISTRO_VERSION
> +security_version:
> + .word CONFIG_SECURITY_VERSION
> +signer:
> + .string CONFIG_SIGNER_NAME
> diff --git a/arch/x86/boot/setup.ld b/arch/x86/boot/setup.ld
> index 96a6c7563538..43ddbaabaf7a 100644
> --- a/arch/x86/boot/setup.ld
> +++ b/arch/x86/boot/setup.ld
> @@ -18,6 +18,7 @@ SECTIONS
> .entrytext : { *(.entrytext) }
> .inittext : { *(.inittext) }
> .initdata : { *(.initdata) }
> + .secdata : { *(.secdata) }
> __end_init = .;
>
> .text : { *(.text) }
> diff --git a/arch/x86/boot/tools/build.c b/arch/x86/boot/tools/build.c
> index 0702d2531bc7..a629d6b615cf 100644
> --- a/arch/x86/boot/tools/build.c
> +++ b/arch/x86/boot/tools/build.c
> @@ -287,7 +287,6 @@ static inline int reserve_pecoff_reloc_section(int c)
> }
> #endif /* CONFIG_EFI_STUB */
>
> -
> /*
> * Parse zoffset.h and find the entry points. We could just #include zoffset.h
> * but that would mean tools/build would have to be rebuilt every time. It's
> diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
> index 07244ea16765..32ffacfaaaff 100644
> --- a/arch/x86/include/uapi/asm/bootparam.h
> +++ b/arch/x86/include/uapi/asm/bootparam.h
> @@ -85,6 +85,7 @@ struct setup_header {
> __u64 pref_address;
> __u32 init_size;
> __u32 handover_offset;
> + __u16 secdata_offset;
> } __attribute__((packed));
>
> struct sys_desc_table {
> --
> 2.12.2
>