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>

6 files changed, 92 insertions, 1 deletions

diff --git a/arch/arm/include/asm/cacheflush.h b/arch/arm/include/asm/cacheflush.h
index 10e78d00a0bb..2d46862e7bef 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 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);
diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index 7a0756df91a2..c9cd9c5bf1e1 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 e6bfe76b2f59..dc2db779cdf4 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -622,9 +622,10 @@ struct section_perm {
         unsigned long end;
         pmdval_t mask;
         pmdval_t prot;
+        pmdval_t clear;
 };
 
-struct section_perm nx_perms[] = {
+static struct section_perm nx_perms[] = {
         /* Make pages tables, etc before _stext RW (set NX). */
         {
                 .start  = PAGE_OFFSET,
@@ -639,8 +640,35 @@ 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
+static 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. Is only
@@ -704,6 +732,24 @@ 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 */