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

[joeyli]

Hi Ard,

On Thu, Jun 01, 2017 at 08:46:26AM +0000, Ard Biesheuvel wrote:
> On 1 June 2017 at 08:11, Gary Lin <glin@xxxxxxxx> wrote:
> > 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?
> >
> 
> This is now entirely x86-specific. My preference would be to have a
> generic solution instead.
> 
> -- 
> Ard.

On x86 platform with secure boot, UEFI boot loader (e.g. shim) keeps the
kernel security version in boot variable for comparing to prevent replay
attack.

Will ARM platform also apply this approach? Does ARM platform have
secure boot knob in firmware?

Thanks a lot!
Joey Lee
 
> 
> >> 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
> >>