Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()

[Mathias Krause]

On 6 April 2017 at 17:59, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Apr 5, 2017 at 5:14 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Wed, Apr 5, 2017 at 4:57 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>> On Wed, Mar 29, 2017 at 6:41 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>>> On Wed, Mar 29, 2017 at 3:38 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>>>> On Wed, Mar 29, 2017 at 11:15 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>>>>> Based on PaX's x86 pax_{open,close}_kernel() implementation, this
>>>>>> allows HAVE_ARCH_RARE_WRITE to work on x86.
>>>>>>
>>>>>> +
>>>>>> +static __always_inline unsigned long __arch_rare_write_begin(void)
>>>>>> +{
>>>>>> +       unsigned long cr0;
>>>>>> +
>>>>>> +       preempt_disable();
>>>>>
>>>>> This looks wrong.  DEBUG_LOCKS_WARN_ON(!irqs_disabled()) would work,
>>>>> as would local_irq_disable().  There's no way that just disabling
>>>>> preemption is enough.
>>>>>
>>>>> (Also, how does this interact with perf nmis?)
>>>>
>>>> Do you mean preempt_disable() isn't strong enough here? I'm open to
>>>> suggestions. The goal would be to make sure nothing between _begin and
>>>> _end would get executed without interruption...
>>>>
>>>
>>> Sorry for the very slow response.
>>>
>>> preempt_disable() isn't strong enough to prevent interrupts, and an
>>> interrupt here would run with WP off, causing unknown havoc.  I tend
>>> to think that the caller should be responsible for turning off
>>> interrupts.
>>
>> So, something like:
>>
>> Top-level functions:
>>
>> static __always_inline rare_write_begin(void)
>> {
>>     preempt_disable();
>>     local_irq_disable();
>>     barrier();
>>     __arch_rare_write_begin();
>>     barrier();
>> }
>
> Looks good, except you don't need preempt_disable().
> local_irq_disable() also disables preemption.  You might need to use
> local_irq_save(), though, depending on whether any callers already
> have IRQs off.

Well, doesn't look good to me. NMIs will still be able to interrupt
this code and will run with CR0.WP = 0.

Shouldn't you instead question yourself why PaX can do it "just" with
preempt_disable() instead?!

Cheers,
Mathias