[PATCH v2 3/3] powerpc/mm: Implement CONFIG_DEBUG_RODATA on PPC32

Christophe Leroy
2017-04-21T13:02:29Z

This patch implements CONFIG_DEBUG_RODATA on PPC32.

As for CONFIG_DEBUG_PAGEALLOC, it deactivates BAT and LTLB mappings
in order to allow page protection setup at the level of each page.

As BAT/LTLB mappings are deactivated, their might be performance
impact. For this reason, we keep it OFF by default.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
v2: For ftrace, only change the attributes of the page to be modified

arch/powerpc/Kconfig.debug | 11 +++++++++++
arch/powerpc/include/asm/pgtable.h | 8 ++++++++
arch/powerpc/kernel/ftrace.c | 8 +++++---
arch/powerpc/mm/init_32.c | 3 ++-
arch/powerpc/mm/pgtable_32.c | 36 ++++++++++++++++++++++++++++++++++++
5 files changed, 62 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
index c86df246339e..047f91564e52 100644
--- a/arch/powerpc/Kconfig.debug
+++ b/arch/powerpc/Kconfig.debug
@@ -369,4 +369,15 @@ config PPC_HTDUMP
def_bool y
depends on PPC_PTDUMP && PPC_BOOK3S

+config DEBUG_RODATA
+ bool "Write protect kernel read-only data structures"
+ depends on DEBUG_KERNEL && PPC32
+ default n
+ help
+ Mark the kernel read-only data as write-protected in the pagetables,
+ in order to catch accidental (and incorrect) writes to such const
+ data. This option may have a performance impact because block
+ mapping via BATs etc... will be disabled.
+ If in doubt, say "N".
+
endmenu
diff --git a/arch/powerpc/include/asm/pgtable.h b/arch/powerpc/include/asm/pgtable.h
index dd01212935ac..142337f3b745 100644
--- a/arch/powerpc/include/asm/pgtable.h
+++ b/arch/powerpc/include/asm/pgtable.h
@@ -80,6 +80,14 @@ unsigned long vmalloc_to_phys(void *vmalloc_addr);

void pgtable_cache_add(unsigned shift, void (*ctor)(void *));
void pgtable_cache_init(void);
+
+#ifdef CONFIG_DEBUG_RODATA
+void set_kernel_text_rw(unsigned long addr);
+void set_kernel_text_ro(unsigned long addr);
+#else
+static inline void set_kernel_text_rw(unsigned long addr) {}
+static inline void set_kernel_text_ro(unsigned long addr) {}
+#endif
#endif /* __ASSEMBLY__ */

#endif /* _ASM_POWERPC_PGTABLE_H */
diff --git a/arch/powerpc/kernel/ftrace.c b/arch/powerpc/kernel/ftrace.c
index 32509de6ce4c..06d2ac53f471 100644
--- a/arch/powerpc/kernel/ftrace.c
+++ b/arch/powerpc/kernel/ftrace.c
@@ -46,6 +46,7 @@ static int
ftrace_modify_code(unsigned long ip, unsigned int old, unsigned int new)
{
unsigned int replaced;
+ int err;

/*
* Note:
@@ -67,10 +68,11 @@ ftrace_modify_code(unsigned long ip, unsigned int old, unsigned int new)
}

/* replace the text with the new text */
- if (patch_instruction((unsigned int *)ip, new))
- return -EPERM;
+ set_kernel_text_rw(ip);
+ err = patch_instruction((unsigned int *)ip, new);
+ set_kernel_text_ro(ip);

- return 0;
+ return err ? -EPERM : 0;
}

/*
diff --git a/arch/powerpc/mm/init_32.c b/arch/powerpc/mm/init_32.c
index 8a7c38b8d335..e39c812b97ca 100644
--- a/arch/powerpc/mm/init_32.c
+++ b/arch/powerpc/mm/init_32.c
@@ -109,7 +109,8 @@ void __init MMU_setup(void)
if (strstr(boot_command_line, "noltlbs")) {
__map_without_ltlbs = 1;
}
- if (debug_pagealloc_enabled()) {
+ if (debug_pagealloc_enabled() ||
+ IS_ENABLED(CONFIG_DEBUG_RODATA)) {
__map_without_bats = 1;
__map_without_ltlbs = 1;
}
diff --git a/arch/powerpc/mm/pgtable_32.c b/arch/powerpc/mm/pgtable_32.c
index 31728f3cdd20..972effec1bb2 100644
--- a/arch/powerpc/mm/pgtable_32.c
+++ b/arch/powerpc/mm/pgtable_32.c
@@ -34,6 +34,7 @@
#include <asm/fixmap.h>
#include <asm/io.h>
#include <asm/setup.h>
+#include <asm/sections.h>

#include "mmu_decl.h"

@@ -375,6 +376,41 @@ void remap_init_ram(void)
change_page_attr(page, numpages, PAGE_KERNEL);
}

+#ifdef CONFIG_DEBUG_RODATA
+void set_kernel_text_rw(unsigned long addr)
+{
+ if (core_kernel_text(addr))
+ change_page_attr(virt_to_page(addr), 1, PAGE_KERNEL_X);
+}
+
+void set_kernel_text_ro(unsigned long addr)
+{
+ if (core_kernel_text(addr))
+ change_page_attr(virt_to_page(addr), 1, PAGE_KERNEL_ROX);
+}
+
+void mark_rodata_ro(void)
+{
+ struct page *page;
+ unsigned long numpages;
+
+ page = virt_to_page(_stext);
+ numpages = PFN_UP((unsigned long)_etext) -
+ PFN_DOWN((unsigned long)_stext);
+
+ change_page_attr(page, numpages, PAGE_KERNEL_ROX);
+ /*
+ * mark .rodata as read only. Use __init_begin rather than __end_rodata
+ * to cover NOTES and EXCEPTION_TABLE.
+ */
+ page = virt_to_page(__start_rodata);
+ numpages = PFN_UP((unsigned long)__init_begin) -
+ PFN_DOWN((unsigned long)__start_rodata);
+
+ change_page_attr(page, numpages, PAGE_KERNEL_RO);
+}
+#endif
+
#ifdef CONFIG_DEBUG_PAGEALLOC
void __kernel_map_pages(struct page *page, int numpages, int enable)
{
--
2.12.0