ARM: mm: allow text and rodata sections to be read-only

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>
Tested-by: Laura Abbott <lauraa@codeaurora.org>
Acked-by: Nicolas Pitre <nico@linaro.org>

3 files changed, 23 insertions, 0 deletions

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 8f75250cbe30..4423a565ef6f 100644
--- a/arch/arm/kernel/machine_kexec.c
+++ b/arch/arm/kernel/machine_kexec.c
@@ -164,6 +164,7 @@ 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;
diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index 18fd68a295ea..3afcb6c2cf06 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);