arm/efi: Split zImage code and data into separate PE/COFF sections

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.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20170818194947.19347-12-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 23 insertions, 7 deletions

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) }