[PATCH v2 08/14] arm64: efi: split Image code and data into separate PE/COFF sections

Ard Biesheuvel
2017-02-08T11:55:41Z

To prevent unintended modifications to the kernel text (malicious or
otherwise) while running the EFI stub, describe the kernel image as
two separate sections: a .text section with read-execute permissions,
covering .text, .rodata and .init.text, and a .data section with
read-write permissions, covering .init.data, .data and .bss.

This relies on the firmware to actually take the section permission
flags into account, but this is something that is currently being
implemented in EDK2, which means we will likely start seeing it in
the wild between one and two years from now.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
arch/arm64/kernel/efi-header.S | 23 +++++++++++++++-----
arch/arm64/kernel/vmlinux.lds.S | 5 +++++
2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kernel/efi-header.S b/arch/arm64/kernel/efi-header.S
index 7637226ea9ca..5870bd611498 100644
--- a/arch/arm64/kernel/efi-header.S
+++ b/arch/arm64/kernel/efi-header.S
@@ -27,8 +27,8 @@ optional_header:
.short PE_OPT_MAGIC_PE32PLUS // PE32+ format
.byte 0x02 // MajorLinkerVersion
.byte 0x14 // MinorLinkerVersion
- .long _end - efi_header_end // SizeOfCode
- .long 0 // SizeOfInitializedData
+ .long __pecoff_data_start - efi_header_end // SizeOfCode
+ .long __pecoff_data_size // SizeOfInitializedData
.long 0 // SizeOfUninitializedData
.long __efistub_entry - _head // AddressOfEntryPoint
.long efi_header_end - _head // BaseOfCode
@@ -74,9 +74,9 @@ extra_header_fields:
// Section table
section_table:
.ascii ".text\0\0\0"
- .long _end - efi_header_end // VirtualSize
+ .long __pecoff_data_start - efi_header_end // VirtualSize
.long efi_header_end - _head // VirtualAddress
- .long _edata - efi_header_end // SizeOfRawData
+ .long __pecoff_data_start - efi_header_end // SizeOfRawData
.long efi_header_end - _head // PointerToRawData

.long 0 // PointerToRelocations
@@ -84,7 +84,20 @@ section_table:
.short 0 // NumberOfRelocations
.short 0 // NumberOfLineNumbers
.long IMAGE_SCN_CNT_CODE | \
- IMAGE_SCN_MEM_EXECUTE | \
+ IMAGE_SCN_MEM_READ | \
+ IMAGE_SCN_MEM_EXECUTE // Characteristics
+
+ .ascii ".data\0\0\0"
+ .long __pecoff_data_size // VirtualSize
+ .long __pecoff_data_start - _head // VirtualAddress
+ .long __pecoff_data_rawsize // SizeOfRawData
+ .long __pecoff_data_start - _head // PointerToRawData
+
+ .long 0 // PointerToRelocations
+ .long 0 // PointerToLineNumbers
+ .short 0 // NumberOfRelocations
+ .short 0 // NumberOfLineNumbers
+ .long IMAGE_SCN_CNT_INITIALIZED_DATA | \
IMAGE_SCN_MEM_READ | \
IMAGE_SCN_MEM_WRITE // Characteristics

diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index b8deffa9e1bf..a93cc2b6f50b 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -149,6 +149,9 @@ SECTIONS
ARM_EXIT_KEEP(EXIT_TEXT)
}

+ . = ALIGN(SZ_4K);
+ __pecoff_data_start = .;
+
.init.data : {
INIT_DATA
INIT_SETUP(16)
@@ -206,6 +209,7 @@ SECTIONS
}

PECOFF_EDATA_PADDING
+ __pecoff_data_rawsize = ABSOLUTE(. - __pecoff_data_start);
_edata = .;

BSS_SECTION(0, 0, 0)
@@ -221,6 +225,7 @@ SECTIONS
. += RESERVED_TTBR0_SIZE;
#endif

+ __pecoff_data_size = ABSOLUTE(. - __pecoff_data_start);
_end = .;

STABS_DEBUG
--
2.7.4