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

[Andy Lutomirski]

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.

--Andy