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

Hoeun Ryu
2017-02-19T10:04:04Z

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