[PATCH] Export mm_update_next_owner function for vhost-net

[gchen.guomin@gmail.com]

From: guomin chen <gchen.guomin@gmail.com>

 Under normal circumstances,When do_exit exits, mm->owner will
 be updated on exit_mm(). but when the kernel process calls
 unuse_mm() and then exits,mm->owner cannot be updated. And it
 will point to a task that has been released.

 Below is my issue on vhost_net:
    A, B are two kernel processes(such as vhost_worker),
    C is a user space process(such as qemu), and all
    three use the mm of the user process C.
    Now, because user process C exits abnormally, the owner of this
    mm becomes A. When A calls unuse_mm and exits, this mm->ower
    still points to the A that has been released.
    When B accesses this mm->owner again, A has been released.

 Process A             Process B
 vhost_worker()        vhost_worker()
  ---------             ---------
  use_mm()              use_mm()
   ...
  unuse_mm()
     tsk->mm=NULL
   do_exit()            page fault
    exit_mm()           access mm->owner
   can't update owner   kernel Oops

                        unuse_mm()

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: kvm@vger.kernel.org
Cc: virtualization@lists.linux-foundation.org
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: guomin chen <gchen.guomin@gmail.com>
---
 drivers/vhost/vhost.c | 1 +
 kernel/exit.c         | 1 +
 2 files changed, 2 insertions(+)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 6b98d8e..7c09087 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -368,6 +368,7 @@ static int vhost_worker(void *data)
 		}
 	}
 	unuse_mm(dev->mm);
+	mm_update_next_owner(dev->mm);
 	set_fs(oldfs);
 	return 0;
 }
diff --git a/kernel/exit.c b/kernel/exit.c
index 0e21e6d..9e046dd 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -486,6 +486,7 @@ void mm_update_next_owner(struct mm_struct *mm)
 	task_unlock(c);
 	put_task_struct(c);
 }
+EXPORT_SYMBOL(mm_update_next_owner);
 #endif /* CONFIG_MEMCG */
 
 /*
-- 
1.8.3.1