Thread (26 messages) 26 messages, 4 authors, 2014-09-04

[PATCH v4 8/8] ARM: mm: allow text and rodata sections to be read-only

From: Will Deacon <hidden>
Date: 2014-08-19 12:35:57
Also in: lkml

On Wed, Aug 13, 2014 at 06:06:33PM +0100, Kees Cook wrote:
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.
[...]
quoted hunk ↗ jump to hunk
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();
I'm afraid I don't understand what you're trying to achieve here. What do
you mean by `clearing a TLB miss'?

[...]
quoted hunk ↗ jump to hunk
diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index ccf392ef40d4..35c838da90d5 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -626,9 +626,10 @@ 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,
@@ -643,8 +644,35 @@ 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.
@@ -713,6 +741,24 @@ 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);
+}
How does this work with LPAE? I don't see a populated clear field there.

Will
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