Date:	Mon, 07 Oct 2013 20:10:35 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Jan Kara <jack@...e.cz>
CC:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 18/26] mm: Convert process_vm_rw_pages() to use get_user_pages_unlocked()

(10/7/13 4:55 PM), Jan Kara wrote:
> On Thu 03-10-13 18:40:06, KOSAKI Motohiro wrote:
>> (10/2/13 3:36 PM), Jan Kara wrote:
>>> On Wed 02-10-13 12:32:33, KOSAKI Motohiro wrote:
>>>> (10/2/13 10:27 AM), Jan Kara wrote:
>>>>> Signed-off-by: Jan Kara <jack@...e.cz>
>>>>> ---
>>>>>    mm/process_vm_access.c | 8 ++------
>>>>>    1 file changed, 2 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c
>>>>> index fd26d0433509..c1bc47d8ed90 100644
>>>>> --- a/mm/process_vm_access.c
>>>>> +++ b/mm/process_vm_access.c
>>>>> @@ -64,12 +64,8 @@ static int process_vm_rw_pages(struct task_struct *task,
>>>>>    	*bytes_copied = 0;
>>>>>
>>>>>    	/* Get the pages we're interested in */
>>>>> -	down_read(&mm->mmap_sem);
>>>>> -	pages_pinned = get_user_pages(task, mm, pa,
>>>>> -				      nr_pages_to_copy,
>>>>> -				      vm_write, 0, process_pages, NULL);
>>>>> -	up_read(&mm->mmap_sem);
>>>>> -
>>>>> +	pages_pinned = get_user_pages_unlocked(task, mm, pa, nr_pages_to_copy,
>>>>> +					       vm_write, 0, process_pages);
>>>>>    	if (pages_pinned != nr_pages_to_copy) {
>>>>>    		rc = -EFAULT;
>>>>>    		goto end;
>>>>
>>>> This is wrong because original code is wrong. In this function, page may
>>>> be pointed to anon pages. Then, you should keep to take mmap_sem until
>>>> finish to copying. Otherwise concurrent fork() makes nasty COW issue.
>>>    Hum, can you be more specific? I suppose you are speaking about situation
>>> when the remote task we are copying data from/to does fork while
>>> process_vm_rw_pages() runs. If we are copying data from remote task, I
>>> don't see how COW could cause any problem. If we are copying to remote task
>>> and fork happens after get_user_pages() but before copy_to_user() then I
>>> can see we might be having some trouble. copy_to_user() would then copy
>>> data into both original remote process and its child thus essentially
>>> bypassing COW. If the child process manages to COW some of the pages before
>>> copy_to_user() happens, it can even see only some of the pages. Is that what
>>> you mean?
>>
>> scenario 1: vm_write==0
>>
>> Process P1 call get_user_pages(pa, process_pages) in process_vm_rw_pages
>> P1 unlock mmap_sem.
>> Process P2 call fork(). and make P3.
>> P2 write memory pa. now the "process_pages" is owned by P3 (the child process)
>> P3 write memory pa. and then the content of "process_pages" is changed.
>> P1 read process_pages as P2's page. but actually, it is P3's data. Then,
>>    P1 observe garbage, at least unintended, data was read.
>    Yeah, this really looks buggy because P1 can see data in (supposedly)
> P2's address space which P2 never wrote there.
>
>> scenario 2: vm_write==1
>>
>> Process P1 call get_user_pages(pa, process_pages) in process_vm_rw_pages.
>> It makes COW break and any anon page sharing broke.
>> P1 unlock mmap_sem.
>> P2 call fork(). and make P3. And, now COW page sharing is restored.
>> P2 write memory pa. now the "process_pages" is owned by P3.
>> P3 write memory pa. it mean P3 changes "process_pages".
>> P1 write process_pages as P2's page. but actually, it is P3's. It
>> override above P3's write and then P3 observe data corruption.
>    Yep, this is a similar problem as above. Thanks for pointing this out.
>
>> The solution is to keep holding mmap_sem until finishing process_pages
>> access because mmap_sem prevent fork. and then race never be happen. I
>> mean you cann't use get_user_pages_unlock() if target address point to
>> anon pages.
>    Yeah, if you are accessing third party mm,

one correction. the condition is,

  - third party mm, or
  - current process have multiple threads and other threads makes fork() and COW break

>you've convinced me you
> currently need mmap_sem to avoid problems with COW on anon pages. I'm just
> thinking that this "hold mmap_sem to prevent fork" is somewhat subtle
> (definitely would deserve a comment) and if it would be needed in more
> places we might be better off if we have a more explicit mechanism for that
> (like a special lock, fork sequence count, or something like that).

Hmm..

Actually, I tried this several years ago. But MM people disliked it because
they prefer faster kernel than simple code. Any additional lock potentially
makes slower the kernel.

However, I fully agree the current mechanism is too complex and misleading,
or at least, very hard to understanding. So, any improvement idea is welcome.

> Anyway
> I'll have this in mind and if I see other places that need this, I'll try
> to come up with some solution. Thanks again for explanation.

NP.

I tried to explain this at MM summit. but my English is too poor and I couldn't
explain enough to you. Sorry about that.


Anyway, I'd like to fix process_vm_rw_pages() before my memory will be flushed again.
Thank you for helping remember this bug.

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