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

[Andy Lutomirski]

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.