Re: [PATCH] ARM: vdso: Mark vDSO code as read-only

[Kees Cook]

On Wed, Feb 17, 2016 at 3:43 PM, David Brown <david.brown@xxxxxxxxxx> wrote:
> On Wed, Feb 17, 2016 at 03:00:52PM -0800, Kees Cook wrote:
>>
>> On Tue, Feb 16, 2016 at 9:20 PM, David Brown <david.brown@xxxxxxxxxx>
>> wrote:
>>>
>>> On Tue, Feb 16, 2016 at 01:52:33PM -0800, Kees Cook wrote:
>>>>
>>>>
>>>> On Tue, Feb 16, 2016 at 1:36 PM, David Brown <david.brown@xxxxxxxxxx>
>>>> wrote:
>>>>>
>>>>>
>>>>> Although the arm vDSO is cleanly separated by code/data with the code
>>>>> being read-only in userspace mappings, the code page is still writable
>>>>> from the kernel.  There have been exploits (such as
>>>>> http://itszn.com/blog/?p=21) that take advantage of this on x86 to go
>>>>> from a bad kernel write to full root.
>>>>>
>>>>> Prevent this specific exploit on arm by putting the vDSO code page in
>>>>> post-init read-only memory as well.
>>>>
>>>>
>>>>
>>>> Is the vdso dynamically built at init time like on x86, or can this
>>>> just use .rodata directly?
>>>
>>>
>>>
>>> On ARM, it is patched during init.  Arm64's is just plain read-only.
>>
>>
>> Okay, great. I've added this to my postinit-readonly series (which I
>> just refreshed and sent out again...)
>
>
> However, this distinction between .rodata and .data..ro_after_init is
> kind of fuzzy, anyway, since they both get made actually read-only at
> the same time (post init).  The patch actually does work fine with the
> vDSO page in .rodata, since the patching happens during init.

Yeah, in the ARM case, that's true. I think we should probably keep it
marked "correctly" though.

> Is there a possible future consideration to perhaps make .rodata read
> only much earlier?

Yeah, this will likely be a future improvement. Some architectures
already mark .rodata before the mark_rodata_ro() call. Once we start
to have more use of postinit-readonly, I suspect we'll see more
clarification of when those things happen.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security