ARM/vdso: Mark the vDSO code read-only after init

Although the ARM vDSO is cleanly separated by code/data with the code
being read-only in userspace mappings, the code page is still writable
from the kernel.

There have been exploits (such as http://itszn.com/blog/?p=21) that
take advantage of this on x86 to go from a bad kernel write to full
root.

Prevent this specific exploit class on ARM as well by putting the vDSO
code page in post-init read-only memory as well.

Before:
	vdso: 1 text pages at base 80927000
	root@Vexpress:/ cat /sys/kernel/debug/kernel_page_tables
	---[ Modules ]---
	---[ Kernel Mapping ]---
	0x80000000-0x80100000           1M     RW NX SHD
	0x80100000-0x80600000           5M     ro x  SHD
	0x80600000-0x80800000           2M     ro NX SHD
	0x80800000-0xbe000000         984M     RW NX SHD

After:
	vdso: 1 text pages at base 8072b000
	root@Vexpress:/ cat /sys/kernel/debug/kernel_page_tables
	---[ Modules ]---
	---[ Kernel Mapping ]---
	0x80000000-0x80100000           1M     RW NX SHD
	0x80100000-0x80600000           5M     ro x  SHD
	0x80600000-0x80800000           2M     ro NX SHD
	0x80800000-0xbe000000         984M     RW NX SHD

Inspired by https://lkml.org/lkml/2016/1/19/494 based on work by the
PaX Team, Brad Spengler, and Kees Cook.

Signed-off-by: David Brown <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brad Spengler <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Emese Revfy <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mathias Krause <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Nathan Lynch <[email protected]>
Cc: PaX Team <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Russell King <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: linux-arch <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

arch/arm/vdso/vdso.S

@@ -23,9 +23,8 @@
 #include <linux/const.h>
 #include <asm/page.h>
 
-	__PAGE_ALIGNED_DATA
-
 	.globl vdso_start, vdso_end
+	.section .data..ro_after_init
 	.balign PAGE_SIZE
 vdso_start:
 	.incbin "arch/arm/vdso/vdso.so"