çå: [PATCH] Export mm_update_next_owner function for vhost-net(Internet mail)

From: guominchen(éåæ)
Date: Thu Dec 13 2018 - 04:20:03 EST

>> 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.

Thank your for taking a look and apologize for my distrub.

>Could you describe how you reproduce this issue?
Sorry, this issue is hard for my to reproduce, But there is such a critical situation.

>I believe vhost process should exit before process C?
Yes, the A, B will exit before C, because usually C will close the open fd and then exit.
However, if C is abnormally exited, such as killed by some fatal signal, A may exit before C

The current issue flow is as follows:
Process C Process A Process B
qemu-system-x86_64: kernel:vhost_net kernel: vhost_net
open /dev/vhost-net
VHOST_SET_OWNER create kthread vhost-%d create kthread vhost-%d
network init use_mm() use_mm()
... ...
Abnormal exited
update mm->owner to A
kthread_should_stop() unuse_mm()
Stop Process A tsk->mm=NULL
can't update owner
A exit completed vhost-%d rcv first package
vhost-%d build rcv buffer for vq
page fault
access mm & mm->owner
NOW,mm->owner still pointer A
kernel NULL pointer at mem_cgroup_from_task()
stop Process B

>> 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)
>> }
>> }
>> unuse_mm(dev->mm);
>> + mm_update_next_owner(dev->mm);

>If you analysis is correct, this is still racy isn't it? (E.g page fault
>happen between unuse_mm() and mm_update_next_owner()).

No, I think this is not racy.
When page fault happend Between unuse_mm() and mm_update_next_owner(), Although tsk->mm =NULL,
But tsk has not exited, So mm->onwer = tsk can still be accessed.

Thanks and regards