Re: [PATCH v2 2/2] ARM: mm: allow text and rodata sections to be read-only

[Kees Cook]

On Wed, Apr 9, 2014 at 2:02 AM, Steve Capper <steve.capper@xxxxxxxxxx> wrote:
> Hi Kees,
>
> On Mon, Apr 07, 2014 at 08:15:10PM -0700, 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. Additional
>> work is needed for kprobes and kexec, so the feature is temporarily
>> marked as unavailable in Kconfig when those options are selected.
>>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>>  arch/arm/include/asm/cacheflush.h |    9 ++++++++
>>  arch/arm/kernel/ftrace.c          |   17 ++++++++++++++
>>  arch/arm/kernel/vmlinux.lds.S     |    3 +++
>>  arch/arm/mm/Kconfig               |   12 ++++++++++
>>  arch/arm/mm/init.c                |   46 +++++++++++++++++++++++++++++++++++++
>>  5 files changed, 87 insertions(+)
>>
>> diff --git a/arch/arm/include/asm/cacheflush.h b/arch/arm/include/asm/cacheflush.h
>> index 8b8b61685a34..b6fea0a1a88b 100644
>> --- a/arch/arm/include/asm/cacheflush.h
>> +++ b/arch/arm/include/asm/cacheflush.h
>> @@ -487,4 +487,13 @@ 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
>> +
>>  #endif
>> diff --git a/arch/arm/kernel/ftrace.c b/arch/arm/kernel/ftrace.c
>> index af9a8a927a4e..ea446ae09c89 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;
>> +}
>
> Would another approach be to keep all the kernel .text ro then override
> probe_kernel_write (which has a weak reference), to create a separate
> temporary rw mapping to the specific page that needs to be modified?
>
> That way you only worry about TLB and cache maintenance for a smaller
> area. Also, your kernel .text VAs never actually become writable, so
> you don't need to worry as much about unauthorised changes whilst your
> guard is temporarily down.
>
> (Though lots of small changes could probably make this more
> expensive, and you will need to double check aliasing in pre-ARMv7).

As I understand it, early boot needs some of these areas RWX. Doing
the protection during init-free means we can avoid all that and still
allow the memory to get reclaimed. As to not doing section
re-mappings, I share the same concern about it being very expensive to
do lots of small changes. As such, I think this is the cleanest
approach that is still portable.

-Kees

-- 
Kees Cook
Chrome OS Security
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