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>

1 files changed, 47 insertions, 1 deletions

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 */