Re: [kernel-hardening] [PATCH v5 3/3] x86/refcount: Implement fast refcount overflow protection

[Kees Cook]

On Thu, Jun 29, 2017 at 7:42 PM, Li Kun <hw.likun@xxxxxxxxxx> wrote:
>
>
> on 2017/6/30 6:05, Kees Cook wrote:
>>
>> On Wed, Jun 28, 2017 at 9:13 PM, Li Kun <hw.likun@xxxxxxxxxx> wrote:
>>>
>>> å 2017/5/31 5:39, Kees Cook åé:
>>>>
>>>> +bool ex_handler_refcount(const struct exception_table_entry *fixup,
>>>> +                        struct pt_regs *regs, int trapnr)
>>>> +{
>>>> +       int reset;
>>>> +
>>>> +       /*
>>>> +        * If we crossed from INT_MAX to INT_MIN, the OF flag (result
>>>> +        * wrapped around) and the SF flag (result is negative) will be
>>>> +        * set. In this case, reset to INT_MAX in an attempt to leave
>>>> the
>>>> +        * refcount usable. Otherwise, we've landed here due to
>>>> producing
>>>> +        * a negative result from either decrementing zero or operating
>>>> on
>>>> +        * a negative value. In this case things are badly broken, so we
>>>> +        * we saturate to INT_MIN / 2.
>>>> +        */
>>>> +       if (regs->flags & (X86_EFLAGS_OF | X86_EFLAGS_SF))
>>>> +               reset = INT_MAX;
>>>
>>>      Should it be like this to indicate that the refcount is wapped from
>>> INT_MAX to INT_MIN ?
>>>          if (regs->flags & (X86_EFLAGS_OF | X86_EFLAGS_SF)
>>>                  == (X86_EFLAGS_OF | X86_EFLAGS_SF))
>>>
>>>                  reset = INT_MAX;
>>
>> Ah yes, thanks for the catch. Yeah, that test is expecting both
>> condition flags to be set.
>>
>> I'm still on the fence about the best way to deal with the bad states.
>> I've been pondering just strictly using a saturation value (INT_MIN /
>> 2), which should offer the same system state protection (except for
>> the inherent resource leak), but that means there isn't really a good
>> way to kill an offending process (since after saturation ALL processes
>> will look like violators). It can be argued that killing the process
>> doesn't actually provide any benefit since the system is still safe,
>> though.
>
> An immature idea,can we set the count to INT_MAX/2 when we detect and kill
> the offending process,
> and wait to see if there will be another offender touching the fence. Er,not
> very acurate,but better than
> killing all the processes doing refcount_add ,i think.
>>>>
>>>> +       else
>>>> +               reset = INT_MIN / 2;
>>>> +       *(int *)regs->cx = reset;

I suppose we could kill a process if it did the wrap from INT_MAX to
INT_MIN, and then ignore (though maintain saturation of) the rest.
i.e. if X86_EFLAGS_OF, kill and saturate. If X86_EFLAGS_SF, saturate.
I'm still curious about catching refcount_dec() (not
refcount_dec_and_test()) hitting zero.

-Kees

-- 
Kees Cook
Pixel Security