Date: Mon, 7 Oct 2013 22:55:08 +0200
From: Jan Kara <jack@suse.cz>
To: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Jan Kara <jack@suse.cz>, LKML <linux-kernel@vger.kernel.org>,
        linux-mm@kvack.org
Subject: Re: [PATCH 18/26] mm: Convert process_vm_rw_pages() to use
 get_user_pages_unlocked()
Message-ID: <20131007205508.GE30441@quack.suse.cz>
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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@suse.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, 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). 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.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR
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