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

[Andy Lutomirski]

On Tue, Jun 21, 2016 at 10:13 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 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. :)
>

Agreed.  There's a big va hole there on x86_64, but I don't know about
other arches.  It might pay to add something to the vmalloc core code.
Any volunteers?

> -Kees
>
> --
> Kees Cook
> Chrome OS & Brillo Security



-- 
Andy Lutomirski
AMA Capital Management, LLC