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

[Gary Lin]

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.

(For more details: https://github.com/lcp/shim/wiki/Security-Version)

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 316152f72bb9..043ff86828a6 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1828,6 +1828,34 @@ 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 2ed8f0c25def..c62e0baf2d89 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -300,7 +300,7 @@ _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
@@ -551,6 +551,7 @@ 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 #####################################################
 
@@ -628,3 +629,14 @@ 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 ddef37b16af2..c46763929a6b 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -84,6 +84,7 @@ struct setup_header {
 	__u64	pref_address;
 	__u32	init_size;
 	__u32	handover_offset;
+	__u16	secdata_offset;
 } __attribute__((packed));
 
 struct sys_desc_table {
-- 
2.13.2