proc: put check_mem_permission after __get_free_page in mem_write

It whould be better if put check_mem_permission after __get_free_page in
mem_write, to be same as function mem_read.

Hugh Dickins explained the reason.

    check_mem_permission gets a reference to the mm.  If we __get_free_page
    after check_mem_permission, imagine what happens if the system is out
    of memory, and the mm we're looking at is selected for killing by the
    OOM killer: while we wait in __get_free_page for more memory, no memory
    is freed from the selected mm because it cannot reach exit_mmap while
    we hold that reference.

Reported-by: Jovi Zhang <bookjovi@gmail.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Stephen Wilson <wilsons@start.ca>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: kosaki

fs/proc/base.c

@@ -894,20 +894,20 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
 	if (!task)
 		goto out_no_task;
 
+	copied = -ENOMEM;
+	page = (char *)__get_free_page(GFP_TEMPORARY);
+	if (!page)
+		goto out_task;
+
 	mm = check_mem_permission(task);
 	copied = PTR_ERR(mm);
 	if (IS_ERR(mm))
-		goto out_task;
+		goto out_free;
 
 	copied = -EIO;
 	if (file->private_data != (void *)((long)current->self_exec_id))
 		goto out_mm;
 
-	copied = -ENOMEM;
-	page = (char *)__get_free_page(GFP_TEMPORARY);
-	if (!page)
-		goto out_mm;
-
 	copied = 0;
 	while (count > 0) {
 		int this_len, retval;
@@ -929,9 +929,11 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
 		count -= retval;			
 	}
 	*ppos = dst;
-	free_page((unsigned long) page);
+
 out_mm:
 	mmput(mm);
+out_free:
+	free_page((unsigned long) page);
 out_task:
 	put_task_struct(task);
 out_no_task: