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

[Kees Cook]

On Wed, Apr 9, 2014 at 12:52 PM, Laura Abbott <lauraa@xxxxxxxxxxxxxx> wrote:
> On 4/9/2014 9:12 AM, Kees Cook wrote:
>> 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.
>>
>
> FWIW, our out of tree patches set up the permissions at map_lowmem time
> and we've never run into any issue with incorrect RWX permissions to
> the best of my knowledge.

I thought there were problems with not being able to free init mem in this case?

> Just for comparison, how many small changes would need to happen for an
> ftrace use case? Would these changes be happening on a hot path?

I'm not familiar with the internals, but it seemed like it was fixing
up a lot of entry points.

-Kees

>
>> -Kees
>>
>
> Thanks,
> Laura
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> hosted by The Linux Foundation



-- 
Kees Cook
Chrome OS Security
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