Re: [PATCH v3 06/13] fork: Add generic vmalloced stack support

[Kees Cook]

On Tue, Jun 21, 2016 at 9:59 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Tue, Jun 21, 2016 at 12:30 AM, Jann Horn <jannh@xxxxxxxxxx> wrote:
>> On Tue, Jun 21, 2016 at 1:43 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>> If CONFIG_VMAP_STACK is selected, kernel stacks are allocated with
>>> vmalloc_node.
>> [...]
>>>  static struct thread_info *alloc_thread_info_node(struct task_struct *tsk,
>>>                                                   int node)
>>>  {
>>> +#ifdef CONFIG_VMAP_STACK
>>> +       struct thread_info *ti = __vmalloc_node_range(
>>> +               THREAD_SIZE, THREAD_SIZE, VMALLOC_START, VMALLOC_END,
>>> +               THREADINFO_GFP | __GFP_HIGHMEM, PAGE_KERNEL,
>>> +               0, node, __builtin_return_address(0));
>>> +
>>
>> After spender gave some hints on IRC about the guard pages not working
>> reliably, I decided to have a closer look at this. As far as I can
>> tell, the idea is that __vmalloc_node_range() automatically adds guard
>> pages unless the VM_NO_GUARD flag is specified. However, those guard
>> pages are *behind* allocations, not in front of them, while a stack
>> guard primarily needs to be in front of the allocation. This wouldn't
>> matter if all allocations in the vmalloc area had guard pages behind
>> them, but if someone first does some data allocation with VM_NO_GUARD
>> and then a stack allocation directly behind that, there won't be a
>> guard between the data allocation and the stack allocation.
>
> I'm tempted to explicitly disallow VM_NO_GUARD in the vmalloc range.
> It has no in-tree users for non-fixed addresses right now.

What about the lack of pre-range guard page? That seems like a
critical feature for this. :)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security