Introduce a linker section and a matching attribute for statically
allocated write rare data. The attribute is named "__wr_after_init".
After the init phase is completed, this section will be modifiable only by
invoking write rare functions.
The section occupies a set of full pages, since the granularity
available for write protection is of one memory page.
The functionality is automatically activated by any architecture that sets
CONFIG_ARCH_HAS_PRMEM
Signed-off-by: Igor Stoppa <igor.stoppa@xxxxxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
CC: Nadav Amit <nadav.amit@xxxxxxxxx>
CC: Matthew Wilcox <willy@xxxxxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Kees Cook <keescook@xxxxxxxxxxxx>
CC: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CC: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
CC: Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxx>
CC: Ahmed Soliman <ahmedsoliman@xxxxxxxxxxx>
CC: linux-integrity@xxxxxxxxxxxxxxx
CC: kernel-hardening@xxxxxxxxxxxxxxxxxx
CC: linux-mm@xxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
---
arch/Kconfig | 15 +++++++++++++++
include/asm-generic/vmlinux.lds.h | 25 +++++++++++++++++++++++++
include/linux/cache.h | 21 +++++++++++++++++++++
init/main.c | 3 +++
4 files changed, 64 insertions(+)
diff --git a/arch/Kconfig b/arch/Kconfig
index 4cfb6de48f79..b0b6d176f1c1 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -808,6 +808,21 @@ 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 c86a1c8f19f4..965e9fbc5452 100644
--- a/init/main.c
+++ b/init/main.c
@@ -496,6 +496,8 @@ void __init __weak thread_stack_cache_init(void)
void __init __weak mem_encrypt_init(void) { }
+void __init __weak wr_init(void) { }
+
bool initcall_debug;
core_param(initcall_debug, initcall_debug, bool, 0644);
@@ -713,6 +715,7 @@ asmlinkage __visible void __init start_kernel(void)
cred_init();
fork_init();
proc_caches_init();
+ wr_init();
uts_ns_init();
buffer_init();
key_init();
--
2.19.1
