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.
[...]
> 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'?
[...]
> 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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