On Tue, Jul 19, 2016 at 1:14 PM, Christian Borntraeger
<borntraeger@xxxxxxxxxx> wrote:
> On 07/19/2016 09:31 PM, Kees Cook wrote:
>> On Tue, Jul 19, 2016 at 2:21 AM, Christian Borntraeger
>> <borntraeger@xxxxxxxxxx> wrote:
>>> On 07/15/2016 11:44 PM, Kees Cook wrote:
>>>> +config HAVE_ARCH_LINEAR_KERNEL_MAPPING
>>>> + bool
>>>> + help
>>>> + An architecture should select this if it has a secondary linear
>>>> + mapping of the kernel text. This is used to verify that kernel
>>>> + text exposures are not visible under CONFIG_HARDENED_USERCOPY.
>>>
>>> I have trouble parsing this. (What does secondary linear mapping mean?)
>>
>> I likely need help clarifying this language...
>>
>>> So let me give an example below
>>>
>>>> +
>>> [...]
>>>> +/* Is this address range in the kernel text area? */
>>>> +static inline const char *check_kernel_text_object(const void *ptr,
>>>> + unsigned long n)
>>>> +{
>>>> + unsigned long textlow = (unsigned long)_stext;
>>>> + unsigned long texthigh = (unsigned long)_etext;
>>>> +
>>>> + if (overlaps(ptr, n, textlow, texthigh))
>>>> + return "<kernel text>";
>>>> +
>>>> +#ifdef HAVE_ARCH_LINEAR_KERNEL_MAPPING
>>>> + /* Check against linear mapping as well. */
>>>> + if (overlaps(ptr, n, (unsigned long)__va(__pa(textlow)),
>>>> + (unsigned long)__va(__pa(texthigh))))
>>>> + return "<linear kernel text>";
>>>> +#endif
>>>> +
>>>> + return NULL;
>>>> +}
>>>
>>> s390 has an address space for user (primary address space from 0..4TB/8PB) and a separate
>>> address space (home space from 0..4TB/8PB) for the kernel. In this home space the kernel
>>> mapping is virtual containing the physical memory as well as vmalloc memory (creating aliases
>>> into the physical one). The kernel text is mapped from _stext to _etext in this mapping.
>>> So I assume this would qualify for HAVE_ARCH_LINEAR_KERNEL_MAPPING ?
>>
>> If I understand your example, yes. In the home space you have two
>> addresses that reference the kernel image?
>
> No, there is only one address that points to the kernel.
> As we have no kernel ASLR yet, and the kernel mapping is
> a 1:1 mapping from 0 to memory end and the kernel is only
> from _stext to _etext. The vmalloc area contains modules
> and vmalloc but not a 2nd kernel mapping.
>
> But thanks for your example, now I understood. If we have only
> one address
>>>> + if (overlaps(ptr, n, textlow, texthigh))
>>>> + return "<kernel text>";
>
> This is just enough.
>
> So what about for the CONFIG text:
>
> An architecture should select this if the kernel mapping has a secondary
> linear mapping of the kernel text - in other words more than one virtual
> kernel address that points to the kernel image. This is used to verify
> that kernel text exposures are not visible under CONFIG_HARDENED_USERCOPY.
Sounds good, I've adjusted it for now.
>> I wonder if I can avoid the CONFIG entirely if I just did a
>> __va(__pa(_stext)) != _stext test... would that break anyone?
>
> Can this be resolved on all platforms at compile time?
Well, I think it still needs a runtime check (compile-time may not be
able to tell about kaslr, or who knows what else). I would really like
to avoid the CONFIG if possible, though. Would this do the right thing
on s390? This appears to work where I'm able to test it (32/64 x86,
32/64 arm):
unsigned long textlow = (unsigned long)_stext;
unsigned long texthigh = (unsigned long)_etext;
unsigned long textlow_linear = (unsigned long)__va(__pa(textlow);
unsigned long texthigh_linear = (unsigned long)__va(__pa(texthigh);
if (overlaps(ptr, n, textlow, texthigh))
return "<kernel text>";
/* Check against possible secondary linear mapping as well. */
if (textlow != textlow_linear &&
overlaps(ptr, n, textlow_linear, texthigh_linear))
return "<linear kernel text>";
return NULL;
-Kees
--
Kees Cook
Chrome OS & Brillo Security
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