[PATCH 02/12] __wr_after_init: linker section and label

Igor Stoppa
2018-12-21T18:14:50Z

Introduce a section and a label for statically allocated write rare
data. The label is named "__wr_after_init".
As the name implies, after the init phase is completed, this section
will be modifiable only by invoking write rare functions.
The section must take up a set of full pages.

To activate both section and label, the arch must set CONFIG_ARCH_HAS_PRMEM

Signed-off-by: Igor Stoppa <igor.stoppa@huawei.com>

CC: Andy Lutomirski <luto@amacapital.net>
CC: Nadav Amit <nadav.amit@gmail.com>
CC: Matthew Wilcox <willy@infradead.org>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Kees Cook <keescook@chromium.org>
CC: Dave Hansen <dave.hansen@linux.intel.com>
CC: Mimi Zohar <zohar@linux.vnet.ibm.com>
CC: Thiago Jung Bauermann <bauerman@linux.ibm.com>
CC: Ahmed Soliman <ahmedsoliman@mena.vt.edu>
CC: linux-integrity@vger.kernel.org
CC: kernel-hardening@lists.openwall.com
CC: linux-mm@kvack.org
CC: linux-kernel@vger.kernel.org
---
arch/Kconfig | 15 +++++++++++++++
include/asm-generic/vmlinux.lds.h | 25 +++++++++++++++++++++++++
include/linux/cache.h | 21 +++++++++++++++++++++
init/main.c | 2 ++
4 files changed, 63 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index e1e540ffa979..8668ffec8098 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -802,6 +802,21 @@ config VMAP_STACK
the stack to map directly to the KASAN shadow map using a formula
that is incorrect if the stack is in vmalloc space.

+config ARCH_HAS_PRMEM
+ def_bool n
+ help
+ architecture specific symbol stating that the architecture provides
+ a back-end function for the write rare operation.
+
+config PRMEM
+ bool "Write protect critical data that doesn't need high write speed."
+ depends on ARCH_HAS_PRMEM
+ default y
+ help
+ If the architecture supports it, statically allocated data which
+ has been selected for hardening becomes (mostly) read-only.
+ The selection happens by labelling the data "__wr_after_init".
+
config ARCH_OPTIONAL_KERNEL_RWX
def_bool n

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 3d7a6a9c2370..ddb1fd608490 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -311,6 +311,30 @@
KEEP(*(__jump_table)) \
__stop___jump_table = .;

+/*
+ * Allow architectures to handle wr_after_init data on their
+ * own by defining an empty WR_AFTER_INIT_DATA.
+ * However, it's important that pages containing WR_RARE data do not
+ * hold anything else, to avoid both accidentally unprotecting something
+ * that is supposed to stay read-only all the time and also to protect
+ * something else that is supposed to be writeable all the time.
+ */
+#ifndef WR_AFTER_INIT_DATA
+#ifdef CONFIG_PRMEM
+#define WR_AFTER_INIT_DATA(align) \
+ . = ALIGN(PAGE_SIZE); \
+ __start_wr_after_init = .; \
+ . = ALIGN(align); \
+ *(.data..wr_after_init) \
+ . = ALIGN(PAGE_SIZE); \
+ __end_wr_after_init = .; \
+ . = ALIGN(align);
+#else
+#define WR_AFTER_INIT_DATA(align) \
+ . = ALIGN(align);
+#endif
+#endif
+
/*
* Allow architectures to handle ro_after_init data on their
* own by defining an empty RO_AFTER_INIT_DATA.
@@ -332,6 +356,7 @@
__start_rodata = .; \
*(.rodata) *(.rodata.*) \
RO_AFTER_INIT_DATA /* Read only after init */ \
+ WR_AFTER_INIT_DATA(align) /* wr after init */ \
KEEP(*(__vermagic)) /* Kernel version magic */ \
. = ALIGN(8); \
__start___tracepoints_ptrs = .; \
diff --git a/include/linux/cache.h b/include/linux/cache.h
index 750621e41d1c..09bd0b9284b6 100644
--- a/include/linux/cache.h
+++ b/include/linux/cache.h
@@ -31,6 +31,27 @@
#define __ro_after_init __attribute__((__section__(".data..ro_after_init")))
#endif

+/*
+ * __wr_after_init is used to mark objects that cannot be modified
+ * directly after init (i.e. after mark_rodata_ro() has been called).
+ * These objects become effectively read-only, from the perspective of
+ * performing a direct write, like a variable assignment.
+ * However, they can be altered through a dedicated function.
+ * It is intended for those objects which are occasionally modified after
+ * init, however they are modified so seldomly, that the extra cost from
+ * the indirect modification is either negligible or worth paying, for the
+ * sake of the protection gained.
+ */
+#ifndef __wr_after_init
+#ifdef CONFIG_PRMEM
+#define __wr_after_init \
+ __attribute__((__section__(".data..wr_after_init")))
+#else
+#define __wr_after_init
+#endif
+#endif
+
+
#ifndef ____cacheline_aligned
#define ____cacheline_aligned __attribute__((__aligned__(SMP_CACHE_BYTES)))
#endif
diff --git a/init/main.c b/init/main.c
index a461150adfb1..a36f2e54f937 100644
--- a/init/main.c
+++ b/init/main.c
@@ -498,6 +498,7 @@ void __init __weak thread_stack_cache_init(void)
void __init __weak mem_encrypt_init(void) { }

void __init __weak poking_init(void) { }
+void __init __weak wr_poking_init(void) { }

bool initcall_debug;
core_param(initcall_debug, initcall_debug, bool, 0644);
@@ -734,6 +735,7 @@ asmlinkage __visible void __init start_kernel(void)
delayacct_init();

poking_init();
+ wr_poking_init();
check_bugs();

acpi_subsystem_init();
--
2.19.1