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x86/boot: Introduce kernel_info 

The relationships between the headers are analogous to the various data
sections:

  setup_header = .data
  boot_params/setup_data = .bss

What is missing from the above list? That's right:

  kernel_info = .rodata

We have been (ab)using .data for things that could go into .rodata or .bss for
a long time, for lack of alternatives and -- especially early on -- inertia.
Also, the BIOS stub is responsible for creating boot_params, so it isn't
available to a BIOS-based loader (setup_data is, though).

setup_header is permanently limited to 144 bytes due to the reach of the
2-byte jump field, which doubles as a length field for the structure, combined
with the size of the "hole" in struct boot_params that a protected-mode loader
or the BIOS stub has to copy it into. It is currently 119 bytes long, which
leaves us with 25 very precious bytes. This isn't something that can be fixed
without revising the boot protocol entirely, breaking backwards compatibility.

boot_params proper is limited to 4096 bytes, but can be arbitrarily extended
by adding setup_data entries. It cannot be used to communicate properties of
the kernel image, because it is .bss and has no image-provided content.

kernel_info solves this by providing an extensible place for information about
the kernel image. It is readonly, because the kernel cannot rely on a
bootloader copying its contents anywhere, but that is OK; if it becomes
necessary it can still contain data items that an enabled bootloader would be
expected to copy into a setup_data chunk.

Do not bump setup_header version in arch/x86/boot/header.S because it
will be followed by additional changes coming into the Linux/x86 boot
protocol.

Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Ross Philipson <ross.philipson@oracle.com>
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: ard.biesheuvel@linaro.org
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: dave.hansen@linux.intel.com
Cc: eric.snowberg@oracle.com
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Juergen Gross <jgross@suse.com>
Cc: kanth.ghatraju@oracle.com
Cc: linux-doc@vger.kernel.org
Cc: linux-efi <linux-efi@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: rdunlap@infradead.org
Cc: ross.philipson@oracle.com
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Cc: xen-devel@lists.xenproject.org
Link: https://lkml.kernel.org/r/20191112134640.16035-2-daniel.kiper@oracle.com
This commit is contained in:
Daniel Kiper 2019-11-12 14:46:38 +01:00 committed by Borislav Petkov
parent c311ed6183
commit 2c33c27fd6
7 changed files with 153 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

126
Documentation/x86/boot.rst
View File

@ -68,8 +68,25 @@ Protocol 2.12 (Kernel 3.8) Added the xloadflags field and extension fields
Protocol 2.13 (Kernel 3.14) Support 32- and 64-bit flags being set in
xloadflags to support booting a 64-bit kernel from 32-bit
EFI
Protocol 2.14: BURNT BY INCORRECT COMMIT ae7e1238e68f2a472a125673ab506d49158c1889
(x86/boot: Add ACPI RSDP address to setup_header)
DO NOT USE!!! ASSUME SAME AS 2.13.
Protocol 2.15: (Kernel 5.5) Added the kernel_info.
============= ============================================================
.. note::
The protocol version number should be changed only if the setup header
is changed. There is no need to update the version number if boot_params
or kernel_info are changed. Additionally, it is recommended to use
xloadflags (in this case the protocol version number should not be
updated either) or kernel_info to communicate supported Linux kernel
features to the boot loader. Due to very limited space available in
the original setup header every update to it should be considered
with great care. Starting from the protocol 2.15 the primary way to
communicate things to the boot loader is the kernel_info.
Memory Layout
=============
@ -207,6 +224,7 @@ Offset/Size Proto Name Meaning
0258/8 2.10+ pref_address Preferred loading address
0260/4 2.10+ init_size Linear memory required during initialization
0264/4 2.11+ handover_offset Offset of handover entry point
0268/4 2.15+ kernel_info_offset Offset of the kernel_info
=========== ======== ===================== ============================================
.. note::
@ -855,6 +873,114 @@ Offset/size: 0x264/4
See EFI HANDOVER PROTOCOL below for more details.
============ ==================
Field name: kernel_info_offset
Type: read
Offset/size: 0x268/4
Protocol: 2.15+
============ ==================
This field is the offset from the beginning of the kernel image to the
kernel_info. The kernel_info structure is embedded in the Linux image
in the uncompressed protected mode region.
The kernel_info
===============
The relationships between the headers are analogous to the various data
sections:
setup_header = .data
boot_params/setup_data = .bss
What is missing from the above list? That's right:
kernel_info = .rodata
We have been (ab)using .data for things that could go into .rodata or .bss for
a long time, for lack of alternatives and -- especially early on -- inertia.
Also, the BIOS stub is responsible for creating boot_params, so it isn't
available to a BIOS-based loader (setup_data is, though).
setup_header is permanently limited to 144 bytes due to the reach of the
2-byte jump field, which doubles as a length field for the structure, combined
with the size of the "hole" in struct boot_params that a protected-mode loader
or the BIOS stub has to copy it into. It is currently 119 bytes long, which
leaves us with 25 very precious bytes. This isn't something that can be fixed
without revising the boot protocol entirely, breaking backwards compatibility.
boot_params proper is limited to 4096 bytes, but can be arbitrarily extended
by adding setup_data entries. It cannot be used to communicate properties of
the kernel image, because it is .bss and has no image-provided content.
kernel_info solves this by providing an extensible place for information about
the kernel image. It is readonly, because the kernel cannot rely on a
bootloader copying its contents anywhere, but that is OK; if it becomes
necessary it can still contain data items that an enabled bootloader would be
expected to copy into a setup_data chunk.
All kernel_info data should be part of this structure. Fixed size data have to
be put before kernel_info_var_len_data label. Variable size data have to be put
after kernel_info_var_len_data label. Each chunk of variable size data has to
be prefixed with header/magic and its size, e.g.:
kernel_info:
.ascii "LToP" /* Header, Linux top (structure). */
.long kernel_info_var_len_data - kernel_info
.long kernel_info_end - kernel_info
.long 0x01234567 /* Some fixed size data for the bootloaders. */
kernel_info_var_len_data:
example_struct: /* Some variable size data for the bootloaders. */
.ascii "0123" /* Header/Magic. */
.long example_struct_end - example_struct
.ascii "Struct"
.long 0x89012345
example_struct_end:
example_strings: /* Some variable size data for the bootloaders. */
.ascii "ABCD" /* Header/Magic. */
.long example_strings_end - example_strings
.asciz "String_0"
.asciz "String_1"
example_strings_end:
kernel_info_end:
This way the kernel_info is self-contained blob.
.. note::
Each variable size data header/magic can be any 4-character string,
without \0 at the end of the string, which does not collide with
existing variable length data headers/magics.
Details of the kernel_info Fields
=================================
============ ========
Field name: header
Offset/size: 0x0000/4
============ ========
Contains the magic number "LToP" (0x506f544c).
============ ========
Field name: size
Offset/size: 0x0004/4
============ ========
This field contains the size of the kernel_info including kernel_info.header.
It does not count kernel_info.kernel_info_var_len_data size. This field should be
used by the bootloaders to detect supported fixed size fields in the kernel_info
and beginning of kernel_info.kernel_info_var_len_data.
============ ========
Field name: size_total
Offset/size: 0x0008/4
============ ========
This field contains the size of the kernel_info including kernel_info.header
and kernel_info.kernel_info_var_len_data.
The Image Checksum
==================

2
arch/x86/boot/Makefile
View File

@ -87,7 +87,7 @@ $(obj)/vmlinux.bin: $(obj)/compressed/vmlinux FORCE
SETUP_OBJS = $(addprefix $(obj)/,$(setup-y))
sed-zoffset := -e 's/^\([0-9a-fA-F]*\) [ABCDGRSTVW] \(startup_32\|startup_64\|efi32_stub_entry\|efi64_stub_entry\|efi_pe_entry\|input_data\|_end\|_ehead\|_text\|z_.*\)$$/\#define ZO_\2 0x\1/p'
sed-zoffset := -e 's/^\([0-9a-fA-F]*\) [ABCDGRSTVW] \(startup_32\|startup_64\|efi32_stub_entry\|efi64_stub_entry\|efi_pe_entry\|input_data\|kernel_info\|_end\|_ehead\|_text\|z_.*\)$$/\#define ZO_\2 0x\1/p'
quiet_cmd_zoffset = ZOFFSET $@
cmd_zoffset = $(NM) $< | sed -n $(sed-zoffset) > $@

4
arch/x86/boot/compressed/Makefile
View File

@ -72,8 +72,8 @@ $(obj)/../voffset.h: vmlinux FORCE
$(obj)/misc.o: $(obj)/../voffset.h
vmlinux-objs-y := $(obj)/vmlinux.lds $(obj)/head_$(BITS).o $(obj)/misc.o \
$(obj)/string.o $(obj)/cmdline.o $(obj)/error.o \
vmlinux-objs-y := $(obj)/vmlinux.lds $(obj)/kernel_info.o $(obj)/head_$(BITS).o \
$(obj)/misc.o $(obj)/string.o $(obj)/cmdline.o $(obj)/error.o \
$(obj)/piggy.o $(obj)/cpuflags.o
vmlinux-objs-$(CONFIG_EARLY_PRINTK) += $(obj)/early_serial_console.o

17
arch/x86/boot/compressed/kernel_info.S Normal file
View File

@ -0,0 +1,17 @@
/* SPDX-License-Identifier: GPL-2.0 */
.section ".rodata.kernel_info", "a"
.global kernel_info
kernel_info:
/* Header, Linux top (structure). */
.ascii "LToP"
/* Size. */
.long kernel_info_var_len_data - kernel_info
/* Size total. */
.long kernel_info_end - kernel_info
kernel_info_var_len_data:
/* Empty for time being... */
kernel_info_end:

1
arch/x86/boot/header.S
View File

@ -567,6 +567,7 @@ 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
kernel_info_offset: .long 0 # Filled in by build.c
# End of setup header #####################################################

5
arch/x86/boot/tools/build.c
View File

@ -56,6 +56,7 @@ u8 buf[SETUP_SECT_MAX*512];
unsigned long efi32_stub_entry;
unsigned long efi64_stub_entry;
unsigned long efi_pe_entry;
unsigned long kernel_info;
unsigned long startup_64;
/*----------------------------------------------------------------------*/
@ -321,6 +322,7 @@ static void parse_zoffset(char *fname)
PARSE_ZOFS(p, efi32_stub_entry);
PARSE_ZOFS(p, efi64_stub_entry);
PARSE_ZOFS(p, efi_pe_entry);
PARSE_ZOFS(p, kernel_info);
PARSE_ZOFS(p, startup_64);
p = strchr(p, '\n');
@ -410,6 +412,9 @@ int main(int argc, char ** argv)
efi_stub_entry_update();
/* Update kernel_info offset. */
put_unaligned_le32(kernel_info, &buf[0x268]);
crc = partial_crc32(buf, i, crc);
if (fwrite(buf, 1, i, dest) != i)
die("Writing setup failed");

1
arch/x86/include/uapi/asm/bootparam.h
View File

@ -88,6 +88,7 @@ struct setup_header {
__u64 pref_address;
__u32 init_size;
__u32 handover_offset;
__u32 kernel_info_offset;
} __attribute__((packed));
struct sys_desc_table {