Re: [PATCH v4 15/29] x86/mm/64: Enable vmapped stacks

[Brian Gerst]

On Mon, Jun 27, 2016 at 11:54 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Mon, Jun 27, 2016 at 8:22 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Mon, Jun 27, 2016 at 8:12 AM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>>> On Mon, Jun 27, 2016 at 11:01 AM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>>>> On Sun, Jun 26, 2016 at 5:55 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>>>  #ifdef CONFIG_X86_64
>>>>>  /* Runs on IST stack */
>>>>>  dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
>>>>>  {
>>>>>         static const char str[] = "double fault";
>>>>>         struct task_struct *tsk = current;
>>>>> +#ifdef CONFIG_VMAP_STACK
>>>>> +       unsigned long cr2;
>>>>> +#endif
>>>>>
>>>>>  #ifdef CONFIG_X86_ESPFIX64
>>>>>         extern unsigned char native_irq_return_iret[];
>>>>> @@ -332,6 +350,20 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
>>>>>         tsk->thread.error_code = error_code;
>>>>>         tsk->thread.trap_nr = X86_TRAP_DF;
>>>>>
>>>>> +#ifdef CONFIG_VMAP_STACK
>>>>> +       /*
>>>>> +        * If we overflow the stack into a guard page, the CPU will fail
>>>>> +        * to deliver #PF and will send #DF instead.  CR2 will contain
>>>>> +        * the linear address of the second fault, which will be in the
>>>>> +        * guard page below the bottom of the stack.
>>>>> +        */
>>>>> +       cr2 = read_cr2();
>>>>> +       if ((unsigned long)tsk->stack - 1 - cr2 < PAGE_SIZE)
>>>>> +               handle_stack_overflow(
>>>>> +                       "kernel stack overflow (double-fault)",
>>>>> +                       regs, cr2);
>>>>> +#endif
>>>>
>>>> Is there any other way to tell if this was from a page fault?  If it
>>>> wasn't a page fault then CR2 is undefined.
>>>
>>> I guess it doesn't really matter, since the fault is fatal either way.
>>> The error message might be incorrect though.
>>>
>>
>> It's at least worth a comment, though.  Maybe I should check if
>> regs->rsp is within 40 bytes of the bottom of the stack, too, such
>> that delivery of an inner fault would have double-faulted assuming the
>> inner fault didn't use an IST vector.
>>
>
> How about:
>
>     /*
>      * If we overflow the stack into a guard page, the CPU will fail
>      * to deliver #PF and will send #DF instead.  CR2 will contain
>      * the linear address of the second fault, which will be in the
>      * guard page below the bottom of the stack.
>      *
>      * We're limited to using heuristics here, since the CPU does
>      * not tell us what type of fault failed and, if the first fault
>      * wasn't a page fault, CR2 may contain stale garbage.  To mostly
>      * rule out garbage, we check if the saved RSP is close enough to
>      * the bottom of the stack to cause exception delivery to fail.
>      * The worst case is 7 stack slots: one for alignment, five for
>      * SS..RIP, and one for the error code.
>      */
>     tsk_stack = (unsigned long)task_stack_page(tsk);
>     if (regs->rsp <= tsk_stack + 7*8 && regs->rsp > tsk_stack - PAGE_SIZE) {
>         /* A double-fault due to #PF delivery failure is plausible. */
>         cr2 = read_cr2();
>         if (tsk_stack - 1 - cr2 < PAGE_SIZE)
>             handle_stack_overflow(
>                 "kernel stack overflow (double-fault)",
>                 regs, cr2);
>     }

I think RSP anywhere in the guard page would be best, since it could
have been decremented by a function prologue into the guard page
before an access that triggers the page fault.

--
Brian Gerst