ARM: 8501/1: mm: flip priority of CONFIG_DEBUG_RODATA

The use of CONFIG_DEBUG_RODATA is generally seen as an essential part of
kernel self-protection:
http://www.openwall.com/lists/kernel-hardening/2015/11/30/13
Additionally, its name has grown to mean things beyond just rodata. To
get ARM closer to this, we ought to rearrange the names of the configs
that control how the kernel protects its memory. What was called
CONFIG_ARM_KERNMEM_PERMS is realy doing the work that other architectures
call CONFIG_DEBUG_RODATA.

This redefines CONFIG_DEBUG_RODATA to actually do the bulk of the
ROing (and NXing). In the place of the old CONFIG_DEBUG_RODATA, use
CONFIG_DEBUG_ALIGN_RODATA, since that's what the option does: adds
section alignment for making rodata explicitly NX, as arm does not split
the page tables like arm64 does without _ALIGN_RODATA.

Also adds human readable names to the sections so I could more easily
debug my typos, and makes CONFIG_DEBUG_RODATA default "y" for CPU_V7.

Results in /sys/kernel/debug/kernel_page_tables for each config state:

 # CONFIG_DEBUG_RODATA is not set
 # CONFIG_DEBUG_ALIGN_RODATA is not set

---[ Kernel Mapping ]---
0x80000000-0x80900000           9M     RW x  SHD
0x80900000-0xa0000000         503M     RW NX SHD

 CONFIG_DEBUG_RODATA=y
 CONFIG_DEBUG_ALIGN_RODATA=y

---[ Kernel Mapping ]---
0x80000000-0x80100000           1M     RW NX SHD
0x80100000-0x80700000           6M     ro x  SHD
0x80700000-0x80a00000           3M     ro NX SHD
0x80a00000-0xa0000000         502M     RW NX SHD

 CONFIG_DEBUG_RODATA=y
 # CONFIG_DEBUG_ALIGN_RODATA is not set

---[ Kernel Mapping ]---
0x80000000-0x80100000           1M     RW NX SHD
0x80100000-0x80a00000           9M     ro x  SHD
0x80a00000-0xa0000000         502M     RW NX SHD

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Laura Abbott <labbott@fedoraproject.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 10 insertions, 9 deletions

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 49bd08178008..53f42508025b 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -572,8 +572,9 @@ void __init mem_init(void)
         }
 }
 
-#ifdef CONFIG_ARM_KERNMEM_PERMS
+#ifdef CONFIG_DEBUG_RODATA
 struct section_perm {
+        const char *name;
         unsigned long start;
         unsigned long end;
         pmdval_t mask;
@@ -584,6 +585,7 @@ struct section_perm {
 static struct section_perm nx_perms[] = {
         /* Make pages tables, etc before _stext RW (set NX). */
         {
+                .name   = "pre-text NX",
                 .start  = PAGE_OFFSET,
                 .end    = (unsigned long)_stext,
                 .mask   = ~PMD_SECT_XN,
@@ -591,14 +593,16 @@ static struct section_perm nx_perms[] = {
         },
         /* Make init RW (set NX). */
         {
+                .name   = "init NX",
                 .start  = (unsigned long)__init_begin,
                 .end    = (unsigned long)_sdata,
                 .mask   = ~PMD_SECT_XN,
                 .prot   = PMD_SECT_XN,
         },
-#ifdef CONFIG_DEBUG_RODATA
+#ifdef CONFIG_DEBUG_ALIGN_RODATA
         /* Make rodata NX (set RO in ro_perms below). */
         {
+                .name   = "rodata NX",
                 .start  = (unsigned long)__start_rodata,
                 .end    = (unsigned long)__init_begin,
                 .mask   = ~PMD_SECT_XN,
@@ -607,10 +611,10 @@ static struct section_perm nx_perms[] = {
 #endif
 };
 
-#ifdef CONFIG_DEBUG_RODATA
 static struct section_perm ro_perms[] = {
         /* Make kernel code and rodata RX (set RO). */
         {
+                .name   = "text/rodata RO",
                 .start  = (unsigned long)_stext,
                 .end    = (unsigned long)__init_begin,
 #ifdef CONFIG_ARM_LPAE
@@ -623,7 +627,6 @@ static struct section_perm ro_perms[] = {
 #endif
         },
 };
-#endif
 
 /*
  * Updates section permissions only for the current mm (sections are
@@ -670,8 +673,8 @@ void set_section_perms(struct section_perm *perms, int n, bool set,
         for (i = 0; i < n; i++) {
                 if (!IS_ALIGNED(perms[i].start, SECTION_SIZE) ||
                     !IS_ALIGNED(perms[i].end, SECTION_SIZE)) {
-                        pr_err("BUG: section %lx-%lx not aligned to %lx\n",
-                                perms[i].start, perms[i].end,
+                        pr_err("BUG: %s section %lx-%lx not aligned to %lx\n",
+                                perms[i].name, perms[i].start, perms[i].end,
                                 SECTION_SIZE);
                         continue;
                 }
@@ -712,7 +715,6 @@ void fix_kernmem_perms(void)
         stop_machine(__fix_kernmem_perms, NULL, NULL);
 }
 
-#ifdef CONFIG_DEBUG_RODATA
 int __mark_rodata_ro(void *unused)
 {
         update_sections_early(ro_perms, ARRAY_SIZE(ro_perms));
@@ -735,11 +737,10 @@ void set_kernel_text_ro(void)
         set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), true,
                                 current->active_mm);
 }
-#endif /* CONFIG_DEBUG_RODATA */
 
 #else
 static inline void fix_kernmem_perms(void) { }
-#endif /* CONFIG_ARM_KERNMEM_PERMS */
+#endif /* CONFIG_DEBUG_RODATA */
 
 void free_tcmmem(void)
 {