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>

1 files changed, 9 insertions, 7 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 0c2c50cc2cca..4ede550517a6 100644
--- a/fs/proc/base.c
+++ b/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: