Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts in do_page_fault()

[Christophe LEROY <christophe.leroy@c-s.fr>]

Le 06/06/2017 à 13:00, Michael Ellerman a écrit :
> christophe leroy <christophe.leroy@c-s.fr> writes:
> 
>> Le 05/06/2017 à 12:45, Michael Ellerman a écrit :
>>> Christophe LEROY <christophe.leroy@c-s.fr> writes:
>>>
>>>> Le 02/06/2017 à 11:26, Michael Ellerman a écrit :
>>>>> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>>>>>
>>>>>> Only the get_user() in store_updates_sp() has to be done outside
>>>>>> the mm semaphore. All the comparison can be done within the semaphore,
>>>>>> so only when really needed.
>>>>>>
>>>>>> As we got a DSI exception, the address pointed by regs->nip is
>>>>>> obviously valid, otherwise we would have had a instruction exception.
>>>>>> So __get_user() can be used instead of get_user()
>>>>>
>>>>> I don't think that part is true.
>>>>>
>>>>> You took a DSI so there *was* an instruction at NIP, but since then it
>>>>> may have been unmapped by another thread.
>>>>>
>>>>> So I don't think you can assume the get_user() will succeed.
>>>>
>>>> The difference between get_user() and __get_user() is that get_user()
>>>> performs an access_ok() in addition.
>>>>
>>>> Doesn't access_ok() only checks whether addr is below TASK_SIZE to
>>>> ensure it is a valid user address ?
>>>
>>> Yeah more or less, via some gross macros.
>>>
>>> I was actually not that worried about the switch from get_user() to
>>> __get_user(), but rather that you removed the check of the return value.
>>> ie.
>>>
>>> -	if (get_user(inst, (unsigned int __user *)regs->nip))
>>> -		return 0;
>>>
>>> Became:
>>>
>>> 	if (is_write && user_mode(regs))
>>> -		store_update_sp = store_updates_sp(regs);
>>> +		__get_user(inst, (unsigned int __user *)regs->nip);
>>>
>>>
>>> I think dropping the access_ok() probably is alright, because the NIP
>>> must (should!) have been in userspace, though as Ben says it's always
>>> good to be paranoid.
>>>
>>> But ignoring that the address can fault at all is wrong AFAICS.
>>
>> I see what you mean now.
>>
>> Indeed,
>>
>> -	unsigned int inst;
>>
>> Became
>>
>> +	unsigned int inst = 0;
>>
>> Since __get_user() doesn't modify 'inst' in case of error, 'inst'
>> remains 0, and store_updates_sp(0) return false. That was the idea behind.
> 
> Ugh. OK, my bad. Though it is a little subtle.
> 
> How about:
> 
> @@ -286,10 +290,13 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>          /*
>           * We want to do this outside mmap_sem, because reading code around nip
>           * can result in fault, which will cause a deadlock when called with
> -        * mmap_sem held
> +        * mmap_sem held. We don't need to check if get_user() fails, if it does
> +        * it won't modify inst, and an inst of 0 will return false from
> +        * store_updates_sp().
>           */
> +       inst = 0;
>          if (is_write && is_user)
> -               store_update_sp = store_updates_sp(regs);
> +               get_user(inst, (unsigned int __user *)regs->nip);
>   
>          if (is_user)
>                  flags |= FAULT_FLAG_USER;
> 
> 
> Then this one can go in.
> 

I just submitted v4 version of the patch "powerpc/mm: Only read faulting 
instruction when necessary in do_page_fault()", skipping this step and 
going directly to the final solution.
The new approach has been to keep everything inside store_updates_sp() 
function and just move the call.

Christophe