[PATCH 8/8] ARM: mm: allow text and rodata sections to be read-only

Kees Cook
2014-08-07T15:03:40Z

This introduces CONFIG_DEBUG_RODATA, making kernel text and rodata
read-only. Additionally, this splits rodata from text so that rodata can
also be NX, which may lead to wasted memory when aligning to SECTION_SIZE.
The read-only areas are made writable during ftrace updates and kexec.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
arch/arm/include/asm/cacheflush.h | 10 +++++++++
arch/arm/kernel/ftrace.c | 19 ++++++++++++++++
arch/arm/kernel/machine_kexec.c | 4 ++--
arch/arm/kernel/vmlinux.lds.S | 3 +++
arch/arm/mm/Kconfig | 12 ++++++++++
arch/arm/mm/init.c | 46 +++++++++++++++++++++++++++++++++++++++
6 files changed, 92 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/cacheflush.h b/arch/arm/include/asm/cacheflush.h
index fd43f7f55b70..0cdf1e31df86 100644
--- a/arch/arm/include/asm/cacheflush.h
+++ b/arch/arm/include/asm/cacheflush.h
@@ -487,6 +487,16 @@ int set_memory_rw(unsigned long addr, int numpages);
int set_memory_x(unsigned long addr, int numpages);
int set_memory_nx(unsigned long addr, int numpages);

+#ifdef CONFIG_DEBUG_RODATA
+void mark_rodata_ro(void);
+void set_kernel_text_rw(void);
+void set_kernel_text_ro(void);
+#else
+static inline void set_kernel_text_rw(void) { }
+static inline void set_kernel_text_ro(void) { }
+#endif
+
void flush_uprobe_xol_access(struct page *page, unsigned long uaddr,
void *kaddr, unsigned long len);
+
#endif
diff --git a/arch/arm/kernel/ftrace.c b/arch/arm/kernel/ftrace.c
index af9a8a927a4e..b8c75e45a950 100644
--- a/arch/arm/kernel/ftrace.c
+++ b/arch/arm/kernel/ftrace.c
@@ -15,6 +15,7 @@
#include <linux/ftrace.h>
#include <linux/uaccess.h>
#include <linux/module.h>
+#include <linux/stop_machine.h>

#include <asm/cacheflush.h>
#include <asm/opcodes.h>
@@ -35,6 +36,22 @@

#define OLD_NOP 0xe1a00000 /* mov r0, r0 */

+static int __ftrace_modify_code(void *data)
+{
+ int *command = data;
+
+ set_kernel_text_rw();
+ ftrace_modify_all_code(*command);
+ set_kernel_text_ro();
+
+ return 0;
+}
+
+void arch_ftrace_update_code(int command)
+{
+ stop_machine(__ftrace_modify_code, &command, NULL);
+}
+
static unsigned long ftrace_nop_replace(struct dyn_ftrace *rec)
{
return rec->arch.old_mcount ? OLD_NOP : NOP;
@@ -73,6 +90,8 @@ int ftrace_arch_code_modify_prepare(void)
int ftrace_arch_code_modify_post_process(void)
{
set_all_modules_text_ro();
+ /* Make sure any TLB misses during machine stop are cleared. */
+ flush_tlb_all();
return 0;
}

diff --git a/arch/arm/kernel/machine_kexec.c b/arch/arm/kernel/machine_kexec.c
index 1ac184f2bdd8..4423a565ef6f 100644
--- a/arch/arm/kernel/machine_kexec.c
+++ b/arch/arm/kernel/machine_kexec.c
@@ -164,11 +164,11 @@ void machine_kexec(struct kimage *image)
reboot_code_buffer = page_address(image->control_code_page);

/* Prepare parameters for reboot_code_buffer*/
+ set_kernel_text_rw();
kexec_start_address = image->start;
kexec_indirection_page = page_list;
kexec_mach_type = machine_arch_type;
- kexec_boot_atags = dt_mem ?: image->start
- - KEXEC_ARM_ZIMAGE_OFFSET
+ kexec_boot_atags = dt_mem ?: image->start - KEXEC_ARM_ZIMAGE_OFFSET
+ KEXEC_ARM_ATAGS_OFFSET;

/* copy our kernel relocation code to the control code page */
diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index a3d07ca2bbb4..542e58919bd9 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -120,6 +120,9 @@ SECTIONS
ARM_CPU_KEEP(PROC_INFO)
}

+#ifdef CONFIG_DEBUG_RODATA
+ . = ALIGN(1<<SECTION_SHIFT);
+#endif
RO_DATA(PAGE_SIZE)

. = ALIGN(4);
diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index 1fc1c30dfdc4..ec706a92dd6b 100644
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -1017,3 +1017,15 @@ config ARM_KERNMEM_PERMS
padded to section-size (1MiB) boundaries (because their permissions
are different and splitting the 1M pages into 4K ones causes TLB
performance problems), wasting memory.
+
+config DEBUG_RODATA
+ bool "Make kernel text and rodata read-only"
+ depends on ARM_KERNMEM_PERMS
+ default y
+ help
+ If this is set, kernel text and rodata will be made read-only. This
+ is to help catch accidental or malicious attempts to change the
+ kernel's executable code. Additionally splits rodata from kernel
+ text so it can be made explicitly non-executable. This creates
+ another section-size padded region, so it can waste more memory
+ space while gaining the read-only protections.
diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index ccf392ef40d4..1a0248a9cfdb 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -626,6 +626,7 @@ struct section_perm {
unsigned long end;
pmdval_t mask;
pmdval_t prot;
+ pmdval_t clear;
};

struct section_perm nx_perms[] = {
@@ -643,8 +644,35 @@ struct section_perm nx_perms[] = {
.mask = ~PMD_SECT_XN,
.prot = PMD_SECT_XN,
},
+#ifdef CONFIG_DEBUG_RODATA
+ /* Make rodata NX (set RO in ro_perms below). */
+ {
+ .start = (unsigned long)__start_rodata,
+ .end = (unsigned long)__init_begin,
+ .mask = ~PMD_SECT_XN,
+ .prot = PMD_SECT_XN,
+ },
+#endif
};

+#ifdef CONFIG_DEBUG_RODATA
+struct section_perm ro_perms[] = {
+ /* Make kernel code and rodata RX (set RO). */
+ {
+ .start = (unsigned long)_stext,
+ .end = (unsigned long)__init_begin,
+#ifdef CONFIG_ARM_LPAE
+ .mask = ~PMD_SECT_RDONLY,
+ .prot = PMD_SECT_RDONLY,
+#else
+ .mask = ~(PMD_SECT_APX | PMD_SECT_AP_WRITE),
+ .prot = PMD_SECT_APX | PMD_SECT_AP_WRITE,
+ .clear = PMD_SECT_AP_WRITE,
+#endif
+ },
+};
+#endif
+
/*
* Updates section permissions only for the current mm (sections are
* copied into each mm). During startup, this is the init_mm.
@@ -713,6 +741,24 @@ static inline void fix_kernmem_perms(void)
{
set_section_perms(nx_perms, prot);
}
+
+#ifdef CONFIG_DEBUG_RODATA
+void mark_rodata_ro(void)
+{
+ set_section_perms(ro_perms, prot);
+}
+
+void set_kernel_text_rw(void)
+{
+ set_section_perms(ro_perms, clear);
+}
+
+void set_kernel_text_ro(void)
+{
+ set_section_perms(ro_perms, prot);
+}
+#endif /* CONFIG_DEBUG_RODATA */
+
#else
static inline void fix_kernmem_perms(void) { }
#endif /* CONFIG_ARM_KERNMEM_PERMS */
--
1.9.1