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Message-ID: <CAKv+Gu8ipMN=NnEs-XVwNBH4dbmt+YQMEbziYTZ8Q7su=V76AA@mail.gmail.com>
Date: Thu, 1 Jun 2017 08:46:26 +0000
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Gary Lin <glin@...e.com>
Cc: "x86@...nel.org" <x86@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Joey Lee <jlee@...e.com>
Subject: Re: [RFC v2 PATCH] x86/boot: Add the secdata section to the setup header
On 1 June 2017 at 08:11, Gary Lin <glin@...e.com> 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.
>> 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@...aro.org>
>> Cc: "H. Peter Anvin" <hpa@...or.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Joey Lee <jlee@...e.com>
>> Signed-off-by: Gary Lin <glin@...e.com>
>> ---
>> 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
>>
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