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x86/boot: Increase section and file alignment to 4k/512
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Align x86 with other EFI architectures, and increase the section
alignment to the EFI page size (4k), so that firmware is able to honour
the section permission attributes and map code read-only and data
non-executable.

There are a number of requirements that have to be taken into account:
- the sign tools get cranky when there are gaps between sections in the
  file view of the image
- the virtual offset of each section must be aligned to the image's
  section alignment
- the file offset *and size* of each section must be aligned to the
  image's file alignment
- the image size must be aligned to the section alignment
- each section's virtual offset must be greater than or equal to the
  size of the headers.

In order to meet all these requirements, while avoiding the need for
lots of padding to accommodate the .compat section, the latter is placed
at an arbitrary offset towards the end of the image, but aligned to the
minimum file alignment (512 bytes). The space before the .text section
is therefore distributed between the PE header, the .setup section and
the .compat section, leaving no gaps in the file coverage, making the
signing tools happy.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230915171623.655440-18-ardb@google.com
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Ard Biesheuvel authored and Ingo Molnar committed Sep 17, 2023
1 parent 34951f3 commit 3e3eabe
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Showing 4 changed files with 51 additions and 125 deletions.
4 changes: 2 additions & 2 deletions arch/x86/boot/compressed/vmlinux.lds.S
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Original file line number Diff line number Diff line change
Expand Up @@ -43,13 +43,13 @@ SECTIONS
*(.rodata.*)
_erodata = . ;
}
.data : {
.data : ALIGN(0x1000) {
_data = . ;
*(.data)
*(.data.*)

/* Add 4 bytes of extra space for a CRC-32 checksum */
. = ALIGN(. + 4, 0x20);
. = ALIGN(. + 4, 0x200);
_edata = . ;
}
. = ALIGN(L1_CACHE_BYTES);
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75 changes: 43 additions & 32 deletions arch/x86/boot/header.S
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Original file line number Diff line number Diff line change
Expand Up @@ -36,6 +36,9 @@ SYSSEG = 0x1000 /* historical load address >> 4 */
#define ROOT_RDONLY 1
#endif

.set salign, 0x1000
.set falign, 0x200

.code16
.section ".bstext", "ax"
#ifdef CONFIG_EFI_STUB
Expand Down Expand Up @@ -82,7 +85,7 @@ optional_header:

.long setup_size + ZO_efi_pe_entry # AddressOfEntryPoint

.long 0x0200 # BaseOfCode
.long setup_size # BaseOfCode
#ifdef CONFIG_X86_32
.long 0 # data
#endif
Expand All @@ -93,8 +96,8 @@ extra_header_fields:
#else
.quad 0 # ImageBase
#endif
.long 0x20 # SectionAlignment
.long 0x20 # FileAlignment
.long salign # SectionAlignment
.long falign # FileAlignment
.word 0 # MajorOperatingSystemVersion
.word 0 # MinorOperatingSystemVersion
.word LINUX_EFISTUB_MAJOR_VERSION # MajorImageVersion
Expand All @@ -103,9 +106,10 @@ extra_header_fields:
.word 0 # MinorSubsystemVersion
.long 0 # Win32VersionValue

.long setup_size + ZO__end # SizeOfImage
.long setup_size + ZO__end + pecompat_vsize
# SizeOfImage

.long 0x200 # SizeOfHeaders
.long salign # SizeOfHeaders
.long 0 # CheckSum
.word IMAGE_SUBSYSTEM_EFI_APPLICATION # Subsystem (EFI application)
#ifdef CONFIG_EFI_DXE_MEM_ATTRIBUTES
Expand Down Expand Up @@ -136,44 +140,51 @@ extra_header_fields:

# Section table
section_table:
#
# The offset & size fields are filled in by build.c.
#
.ascii ".setup"
.byte 0
.byte 0
.long 0
.long 0x0 # startup_{32,64}
.long 0 # Size of initialized data
# on disk
.long 0x0 # startup_{32,64}
.long 0 # PointerToRelocations
.long 0 # PointerToLineNumbers
.word 0 # NumberOfRelocations
.word 0 # NumberOfLineNumbers
.long IMAGE_SCN_CNT_CODE | \
.long setup_size - salign # VirtualSize
.long salign # VirtualAddress
.long pecompat_fstart - salign # SizeOfRawData
.long salign # PointerToRawData

.long 0, 0, 0
.long IMAGE_SCN_CNT_INITIALIZED_DATA | \
IMAGE_SCN_MEM_READ | \
IMAGE_SCN_MEM_EXECUTE # Characteristics
IMAGE_SCN_MEM_DISCARDABLE # Characteristics

#ifdef CONFIG_EFI_MIXED
#
# The offset & size fields are filled in by build.c.
#
.asciz ".compat"
.long 0
.long 0x0
.long 0 # Size of initialized data
# on disk
.long 0x0
.long 0 # PointerToRelocations
.long 0 # PointerToLineNumbers
.word 0 # NumberOfRelocations
.word 0 # NumberOfLineNumbers

.long 8 # VirtualSize
.long setup_size + ZO__end # VirtualAddress
.long pecompat_fsize # SizeOfRawData
.long pecompat_fstart # PointerToRawData

.long 0, 0, 0
.long IMAGE_SCN_CNT_INITIALIZED_DATA | \
IMAGE_SCN_MEM_READ | \
IMAGE_SCN_MEM_DISCARDABLE # Characteristics
#endif

/*
* Put the IA-32 machine type and the associated entry point address in
* the .compat section, so loaders can figure out which other execution
* modes this image supports.
*/
.pushsection ".pecompat", "a", @progbits
.balign falign
.set pecompat_vsize, salign
.globl pecompat_fstart
pecompat_fstart:
.byte 0x1 # Version
.byte 8 # Size
.word IMAGE_FILE_MACHINE_I386 # PE machine type
.long setup_size + ZO_efi32_pe_entry # Entrypoint
.popsection
#else
.set pecompat_vsize, 0
.set pecompat_fstart, setup_size
#endif
.ascii ".text"
.byte 0
.byte 0
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7 changes: 4 additions & 3 deletions arch/x86/boot/setup.ld
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Original file line number Diff line number Diff line change
Expand Up @@ -36,16 +36,17 @@ SECTIONS
. = ALIGN(16);
.data : { *(.data*) }

.pecompat : { *(.pecompat) }
PROVIDE(pecompat_fsize = setup_size - pecompat_fstart);

.signature : {
setup_sig = .;
LONG(0x5a5aaa55)

/* Reserve some extra space for the compat section */
setup_size = ALIGN(ABSOLUTE(.) + 32, 512);
setup_size = ALIGN(ABSOLUTE(.), 4096);
setup_sects = ABSOLUTE(setup_size / 512);
}


. = ALIGN(16);
.bss :
{
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90 changes: 2 additions & 88 deletions arch/x86/boot/tools/build.c
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Original file line number Diff line number Diff line change
Expand Up @@ -47,9 +47,6 @@ typedef unsigned int u32;
/* This must be large enough to hold the entire setup */
u8 buf[SETUP_SECT_MAX*512];

#define PECOFF_COMPAT_RESERVE 0x20

static unsigned long efi32_pe_entry;
static unsigned long _edata;

/*----------------------------------------------------------------------*/
Expand Down Expand Up @@ -136,85 +133,6 @@ static void usage(void)
die("Usage: build setup system zoffset.h image");
}

#ifdef CONFIG_EFI_STUB

static void update_pecoff_section_header_fields(char *section_name, u32 vma, u32 size, u32 datasz, u32 offset)
{
unsigned int pe_header;
unsigned short num_sections;
u8 *section;

pe_header = get_unaligned_le32(&buf[0x3c]);
num_sections = get_unaligned_le16(&buf[pe_header + 6]);

#ifdef CONFIG_X86_32
section = &buf[pe_header + 0xa8];
#else
section = &buf[pe_header + 0xb8];
#endif

while (num_sections > 0) {
if (strncmp((char*)section, section_name, 8) == 0) {
/* section header size field */
put_unaligned_le32(size, section + 0x8);

/* section header vma field */
put_unaligned_le32(vma, section + 0xc);

/* section header 'size of initialised data' field */
put_unaligned_le32(datasz, section + 0x10);

/* section header 'file offset' field */
put_unaligned_le32(offset, section + 0x14);

break;
}
section += 0x28;
num_sections--;
}
}

static void update_pecoff_section_header(char *section_name, u32 offset, u32 size)
{
update_pecoff_section_header_fields(section_name, offset, size, size, offset);
}

static void update_pecoff_setup(unsigned int size)
{
u32 setup_offset = 0x200;
u32 compat_offset = size - PECOFF_COMPAT_RESERVE;
u32 setup_size = compat_offset - setup_offset;

update_pecoff_section_header(".setup", setup_offset, setup_size);

#ifdef CONFIG_EFI_MIXED
update_pecoff_section_header(".compat", compat_offset, PECOFF_COMPAT_RESERVE);

/*
* Put the IA-32 machine type (0x14c) and the associated entry point
* address in the .compat section, so loaders can figure out which other
* execution modes this image supports.
*/
buf[compat_offset] = 0x1;
buf[compat_offset + 1] = 0x8;
put_unaligned_le16(0x14c, &buf[compat_offset + 2]);
put_unaligned_le32(efi32_pe_entry + size, &buf[compat_offset + 4]);
#endif
}

#else

static inline void update_pecoff_setup(unsigned int size) {}

#endif /* CONFIG_EFI_STUB */

static int reserve_pecoff_compat_section(int c)
{
/* Reserve 0x20 bytes for .compat section */
memset(buf+c, 0, PECOFF_COMPAT_RESERVE);
return PECOFF_COMPAT_RESERVE;
}

/*
* 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
Expand Down Expand Up @@ -243,7 +161,6 @@ static void parse_zoffset(char *fname)
p = (char *)buf;

while (p && *p) {
PARSE_ZOFS(p, efi32_pe_entry);
PARSE_ZOFS(p, _edata);

p = strchr(p, '\n');
Expand Down Expand Up @@ -283,17 +200,14 @@ int main(int argc, char ** argv)
die("Boot block hasn't got boot flag (0xAA55)");
fclose(file);

c += reserve_pecoff_compat_section(c);

/* Pad unused space with zeros */
setup_sectors = (c + 511) / 512;
setup_sectors = (c + 4095) / 4096;
setup_sectors *= 8;
if (setup_sectors < SETUP_SECT_MIN)
setup_sectors = SETUP_SECT_MIN;
i = setup_sectors*512;
memset(buf+c, 0, i-c);

update_pecoff_setup(i);

/* Open and stat the kernel file */
fd = open(argv[2], O_RDONLY);
if (fd < 0)
Expand Down

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