[PATCH] Export mm_update_next_owner function for vhost-net

From: gchen . guomin
Date: Wed Dec 12 2018 - 23:47:52 EST

From: guomin chen <gchen.guomin@xxxxxxxxx>

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()
do_exit() page fault
exit_mm() access mm->owner
can't update owner kernel Oops


Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
Cc: Jason Wang <jasowang@xxxxxxxxxx>
Cc: kvm@xxxxxxxxxxxxxxx
Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx>
Cc: "Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>
Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: guomin chen <gchen.guomin@xxxxxxxxx>
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)
+ mm_update_next_owner(dev->mm);
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)
#endif /* CONFIG_MEMCG */