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Subject[PATCH 11/14] arm: efi: split zImage code and data into separate PE/COFF sections
Date
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, .piggytext and the GOT sections (which the
stub does not care about anyway), and a .data section with read-write
permissions, covering .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.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
arch/arm/boot/compressed/efi-header.S | 32 +++++++++++++++++++++++---------
arch/arm/boot/compressed/vmlinux.lds.S | 30 +++++++++++++++++++++++-------
2 files changed, 46 insertions(+), 16 deletions(-)

diff --git a/arch/arm/boot/compressed/efi-header.S b/arch/arm/boot/compressed/efi-header.S
index 542e1ad432ae..c94a88ae834d 100644
--- a/arch/arm/boot/compressed/efi-header.S
+++ b/arch/arm/boot/compressed/efi-header.S
@@ -54,20 +54,22 @@ coff_header:
IMAGE_FILE_EXECUTABLE_IMAGE | \
IMAGE_FILE_LINE_NUMS_STRIPPED @ Characteristics

+#define __pecoff_code_size (__pecoff_data_start - __efi_start)
+
optional_header:
.short PE_OPT_MAGIC_PE32 @ PE32 format
.byte 0x02 @ MajorLinkerVersion
.byte 0x14 @ MinorLinkerVersion
- .long _end - __efi_start @ SizeOfCode
- .long 0 @ SizeOfInitializedData
+ .long __pecoff_code_size @ SizeOfCode
+ .long __pecoff_data_size @ SizeOfInitializedData
.long 0 @ SizeOfUninitializedData
.long efi_stub_entry - start @ AddressOfEntryPoint
.long start_offset @ BaseOfCode
- .long 0 @ BaseOfData
+ .long __pecoff_data_start - start @ BaseOfData

extra_header_fields:
.long 0 @ ImageBase
- .long SZ_512 @ SectionAlignment
+ .long SZ_4K @ SectionAlignment
.long SZ_512 @ FileAlignment
.short 0 @ MajorOsVersion
.short 0 @ MinorOsVersion
@@ -77,7 +79,7 @@ extra_header_fields:
.short 0 @ MinorSubsystemVersion
.long 0 @ Win32VersionValue

- .long _end - start @ SizeOfImage
+ .long __pecoff_end - start @ SizeOfImage
.long start_offset @ SizeOfHeaders
.long 0 @ CheckSum
.short IMAGE_SUBSYSTEM_EFI_APPLICATION @ Subsystem
@@ -98,9 +100,9 @@ extra_header_fields:

section_table:
.ascii ".text\0\0\0"
- .long _end - __efi_start @ VirtualSize
+ .long __pecoff_code_size @ VirtualSize
.long __efi_start @ VirtualAddress
- .long _edata - __efi_start @ SizeOfRawData
+ .long __pecoff_code_size @ SizeOfRawData
.long __efi_start @ PointerToRawData
.long 0 @ PointerToRelocations
.long 0 @ PointerToLineNumbers
@@ -108,12 +110,24 @@ section_table:
.short 0 @ NumberOfLineNumbers
.long IMAGE_SCN_CNT_CODE | \
IMAGE_SCN_MEM_READ | \
- IMAGE_SCN_MEM_WRITE | \
IMAGE_SCN_MEM_EXECUTE @ Characteristics

+ .ascii ".data\0\0\0"
+ .long __pecoff_data_size @ VirtualSize
+ .long __pecoff_data_start - start @ VirtualAddress
+ .long __pecoff_data_rawsize @ SizeOfRawData
+ .long __pecoff_data_start - start @ 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
+
.set section_count, (. - section_table) / 40

- .align 9
+ .align 12
__efi_start:
#endif
.endm
diff --git a/arch/arm/boot/compressed/vmlinux.lds.S b/arch/arm/boot/compressed/vmlinux.lds.S
index 81c493156ce8..7a4c59154361 100644
--- a/arch/arm/boot/compressed/vmlinux.lds.S
+++ b/arch/arm/boot/compressed/vmlinux.lds.S
@@ -48,13 +48,6 @@ SECTIONS
*(.rodata)
*(.rodata.*)
}
- .data : {
- /*
- * The EFI stub always executes from RAM, and runs strictly before the
- * decompressor, so we can make an exception for its r/w data, and keep it
- */
- *(.data.efistub)
- }
.piggydata : {
*(.piggydata)
}
@@ -70,6 +63,26 @@ SECTIONS
/* ensure the zImage file size is always a multiple of 64 bits */
/* (without a dummy byte, ld just ignores the empty section) */
.pad : { BYTE(0); . = ALIGN(8); }
+
+#ifdef CONFIG_EFI_STUB
+ .data : ALIGN(4096) {
+ __pecoff_data_start = .;
+ /*
+ * The EFI stub always executes from RAM, and runs strictly before the
+ * decompressor, so we can make an exception for its r/w data, and keep it
+ */
+ *(.data.efistub)
+ __pecoff_data_end = .;
+
+ /*
+ * PE/COFF mandates a file size which is a multiple of 512 bytes if the
+ * section size equals or exceeds 4 KB
+ */
+ . = ALIGN(512);
+ }
+ __pecoff_data_rawsize = . - ADDR(.data);
+#endif
+
_edata = .;

_magic_sig = ZIMAGE_MAGIC(0x016f2818);
@@ -84,6 +97,9 @@ SECTIONS
. = ALIGN(8); /* the stack must be 64-bit aligned */
.stack : { *(.stack) }

+ PROVIDE(__pecoff_data_size = ALIGN(512) - ADDR(.data));
+ PROVIDE(__pecoff_end = ALIGN(512));
+
.stab 0 : { *(.stab) }
.stabstr 0 : { *(.stabstr) }
.stab.excl 0 : { *(.stab.excl) }
--
2.11.0
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