[PATCH] ARM: mm: flip priority of CONFIG_DEBUG_RODATA

[Kees Cook <keescook@chromium.org>]

Given the choice between making things NX or making things RO, we want
RO first. As such, redefine CONFIG_DEBUG_RODATA to actually do the bulk
of the ROing (and NXing). In the place of the old CONFIG_DEBUG_RODATA,
introduce CONFIG_DEBUG_RODATA_ALIGN (after arm64's config that does the
same thing) to add the additional section alignment for making rodata
explicitly NX. 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>
---
 arch/arm/kernel/vmlinux.lds.S | 10 +++++-----
 arch/arm/mm/Kconfig           | 34 ++++++++++++++++++----------------
 arch/arm/mm/init.c            | 18 ++++++++++--------
 3 files changed, 33 insertions(+), 29 deletions(-)

diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index 8b60fde5ce48..a6e395c53a48 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -8,7 +8,7 @@
 #include <asm/thread_info.h>
 #include <asm/memory.h>
 #include <asm/page.h>
-#ifdef CONFIG_ARM_KERNMEM_PERMS
+#ifdef CONFIG_DEBUG_RODATA
 #include <asm/pgtable.h>
 #endif
 
@@ -94,7 +94,7 @@ SECTIONS
 		HEAD_TEXT
 	}
 
-#ifdef CONFIG_ARM_KERNMEM_PERMS
+#ifdef CONFIG_DEBUG_RODATA
 	. = ALIGN(1<<SECTION_SHIFT);
 #endif
 
@@ -117,7 +117,7 @@ SECTIONS
 			ARM_CPU_KEEP(PROC_INFO)
 	}
 
-#ifdef CONFIG_DEBUG_RODATA
+#ifdef CONFIG_DEBUG_ALIGN_RODATA
 	. = ALIGN(1<<SECTION_SHIFT);
 #endif
 	RO_DATA(PAGE_SIZE)
@@ -153,7 +153,7 @@ SECTIONS
 	_etext = .;			/* End of text and rodata section */
 
 #ifndef CONFIG_XIP_KERNEL
-# ifdef CONFIG_ARM_KERNMEM_PERMS
+# ifdef CONFIG_DEBUG_RODATA
 	. = ALIGN(1<<SECTION_SHIFT);
 # else
 	. = ALIGN(PAGE_SIZE);
@@ -231,7 +231,7 @@ SECTIONS
 	__data_loc = ALIGN(4);		/* location in binary */
 	. = PAGE_OFFSET + TEXT_OFFSET;
 #else
-#ifdef CONFIG_ARM_KERNMEM_PERMS
+#ifdef CONFIG_DEBUG_RODATA
 	. = ALIGN(1<<SECTION_SHIFT);
 #else
 	. = ALIGN(THREAD_SIZE);
diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index 41218867a9a6..b617084e9520 100644
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -1039,24 +1039,26 @@ config ARCH_SUPPORTS_BIG_ENDIAN
 	  This option specifies the architecture can support big endian
 	  operation.
 
-config ARM_KERNMEM_PERMS
-	bool "Restrict kernel memory permissions"
+config DEBUG_RODATA
+	bool "Make kernel text and rodata read-only"
 	depends on MMU
+	default y if CPU_V7
 	help
-	  If this is set, kernel memory other than kernel text (and rodata)
-	  will be made non-executable. The tradeoff is that each region is
-	  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.
+	  If this is set, kernel memory (text, rodata, etc) will be made
+	  read-only, and non-text kernel memory will be made non-executable.
+	  The tradeoff is that each region is padded to section-size (1MiB)
+	  boundaries (because their permissions are different and splitting
+	  the 1M pages into 4K ones causes TLB performance problems), which
+	  can waste memory.
 
-config DEBUG_RODATA
-	bool "Make kernel text and rodata read-only"
-	depends on ARM_KERNMEM_PERMS
+config DEBUG_ALIGN_RODATA
+	bool "Make rodata strictly non-executable"
+	depends on DEBUG_RODATA
 	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.
+	  If this is set, rodata will be made explicitly non-executable. This
+	  provides protection on the rare chance that attackers might find and
+	  use ROP gadgets that exist in the rodata section. This adds an
+	  additional section-aligned split of rodata from kernel text so it
+	  can be made explicitly non-executable. This padding may waste memory
+	  space to gain this additional protection.
diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 8a63b4cdc0f2..e99f65fbcf2b 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -568,8 +568,9 @@ 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;
@@ -580,6 +581,7 @@ 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,
@@ -587,14 +589,16 @@ 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,
@@ -603,10 +607,10 @@ 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
@@ -619,7 +623,6 @@ static struct section_perm ro_perms[] = {
 #endif
 	},
 };
-#endif
 
 /*
  * Updates section permissions only for the current mm (sections are
@@ -666,7 +669,8 @@ static inline bool arch_has_strict_perms(void)
 	for (i = 0; i < ARRAY_SIZE(perms); 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", \
+			pr_err("BUG: %s section %lx-%lx not aligned to %lx\n", \
+				perms[i].name,				\
 				perms[i].start, perms[i].end,		\
 				SECTION_SIZE);				\
 			continue;					\
@@ -685,7 +689,6 @@ 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);
@@ -700,11 +703,10 @@ 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 */
+#endif /* CONFIG_DEBUG_RODATA */
 
 void free_tcmmem(void)
 {
-- 
1.9.1


-- 
Kees Cook
Chrome OS & Brillo Security