[PATCH v6 8/8] ARM: mm: allow text and rodata sections to be read-only
From: Kees Cook
Date: Thu Sep 18 2014 - 15:23:10 EST
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@xxxxxxxxxxxx>
Tested-by: Laura Abbott <lauraa@xxxxxxxxxxxxxx>
Acked-by: Nicolas Pitre <nico@xxxxxxxxxx>
---
arch/arm/include/asm/cacheflush.h | 10 ++++++++
arch/arm/kernel/ftrace.c | 19 ++++++++++++++++
arch/arm/kernel/machine_kexec.c | 1 +
arch/arm/kernel/vmlinux.lds.S | 3 +++
arch/arm/mm/Kconfig | 12 ++++++++++
arch/arm/mm/init.c | 48 ++++++++++++++++++++++++++++++++++++++-
6 files changed, 92 insertions(+), 1 deletion(-)
diff --git a/arch/arm/include/asm/cacheflush.h b/arch/arm/include/asm/cacheflush.h
index 79ecb4f34ffb..9108292edcb5 100644
--- a/arch/arm/include/asm/cacheflush.h
+++ b/arch/arm/include/asm/cacheflush.h
@@ -486,6 +486,16 @@ 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 a3d07ca2bbb4..542e58919bd9 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 */
--
1.9.1
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