[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
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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.
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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'? [...]
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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