[kernel-hardening] [RFC 1/7] arch: add __ro_mostly_after_init section marker

[Hoeun Ryu <hoeun.ryu@gmail.com>]

 After `__ro_after_init` marker is included in kernel, many kernel data
objects can be read-only-after-init. But there are many other places that
would be good to read-only-after-init but `__ro_after_init` can not be simply
applicable to them because they should be writable at some points, which are
during module_init/exit or dynamic de/registration for a specific subsystem.
 `__ro_mostly_after_init` is basically the same to `__ro_after_init`. The
section is mapped as read-only after kernel init. The different thing is
this section is temporarily mapped as read-write during module_init/exit and
de/registration of a subsystem using set_ro_mostly_after_init_rw/ro pair.
 Use `__ro_mostly_after_init` as a way to mark such memory instead when
`__ro_after_init` is not applicable because the memory should be writable
at the described points of time. They are read-only right after kernel init
and writable temporarily only during module_init/exit and dynamic
de/registration for a subsystem.

Signed-off-by: Hoeun Ryu <hoeun.ryu@gmail.com>
---
 include/asm-generic/sections.h    |  1 +
 include/asm-generic/vmlinux.lds.h | 10 ++++++++++
 include/linux/cache.h             | 11 +++++++++++
 3 files changed, 22 insertions(+)

diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
index 4df64a1..16a6f21 100644
--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h
@@ -34,6 +34,7 @@ extern char __bss_start[], __bss_stop[];
 extern char __init_begin[], __init_end[];
 extern char _sinittext[], _einittext[];
 extern char __start_data_ro_after_init[], __end_data_ro_after_init[];
+extern char __start_data_ro_mostly_after_init[], __end_data_ro_mostly_after_init[];
 extern char _end[];
 extern char __per_cpu_load[], __per_cpu_start[], __per_cpu_end[];
 extern char __kprobes_text_start[], __kprobes_text_end[];
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 4e09b28..cc5f44e 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -265,6 +265,15 @@
 	__end_data_ro_after_init = .;
 #endif
 
+#ifndef RO_MOSTLY_AFTER_INIT_DATA
+#define RO_MOSTLY_AFTER_INIT_DATA(align)				\
+	. = ALIGN(align);						\
+	VMLINUX_SYMBOL(__start_data_ro_mostly_after_init) = .;		\
+	*(.data..ro_mostly_after_init)					\
+	. = ALIGN(align);						\
+	VMLINUX_SYMBOL(__end_data_ro_mostly_after_init) = .;
+#endif
+
 /*
  * Read only Data
  */
@@ -275,6 +284,7 @@
 		*(.rodata) *(.rodata.*)					\
 		RO_AFTER_INIT_DATA	/* Read only after init */	\
 		KEEP(*(__vermagic))	/* Kernel version magic */	\
+		RO_MOSTLY_AFTER_INIT_DATA(align)			\
 		. = ALIGN(8);						\
 		VMLINUX_SYMBOL(__start___tracepoints_ptrs) = .;		\
 		KEEP(*(__tracepoints_ptrs)) /* Tracepoints: pointer array */ \
diff --git a/include/linux/cache.h b/include/linux/cache.h
index 1be04f8..fd1cb9b 100644
--- a/include/linux/cache.h
+++ b/include/linux/cache.h
@@ -30,6 +30,17 @@
 #define __ro_after_init __attribute__((__section__(".data..ro_after_init")))
 #endif
 
+/*
+ * __ro_mostly_after_init is almost like __ro_after_init.
+ * but __ro_mostly_after_init section is temporarily writable only during
+ * module_init/exit or dynamic de/registeration of a subsystem using
+ * set_ro_mostly_after_init_rw/ro pair.
+ */
+#ifndef __ro_mostly_after_init
+#define __ro_mostly_after_init \
+	__attribute__((__section__(".data..ro_mostly_after_init")))
+#endif
+
 #ifndef ____cacheline_aligned
 #define ____cacheline_aligned __attribute__((__aligned__(SMP_CACHE_BYTES)))
 #endif
-- 
2.7.4