linux-kernel - Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts in do_page

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Date:   Mon, 5 Jun 2017 19:48:16 +0200
From:   christophe leroy <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts in
 do_page_fault()



Le 05/06/2017 à 12:45, Michael Ellerman a écrit :
> Christophe LEROY <christophe.leroy@....fr> writes:
>
>> Le 02/06/2017 à 11:26, Michael Ellerman a écrit :
>>> Christophe Leroy <christophe.leroy@....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.

Christophe

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