Re: [PATCH] Fix mm->owner point to a tsk that has been free

[Michal Hocko]

On Tue 18-12-18 13:24:44, gchen.guomin@xxxxxxxxx wrote:
> From: guomin chen <gchen.guomin@xxxxxxxxx>
> 
> When mm->owner is modified by exit_mm, if the new owner directly calls
> unuse_mm to exit, it will cause Use-After-Free. Due to the unuse_mm()
> directly sets tsk->mm=NULL.
> 
>  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.
> 
> The current issue flow is as follows: (Process A,B,C use the same mm)
> 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
>    ...
>   do_exit
>   exit_mm()
>   update mm->owner to A
>   exit_files()
>    close_files()
>    kthread_should_stop() unuse_mm()
>     Stop Process A       tsk->mm=NULL
>                          do_exit()
>                          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 UAF
>     stop Process B
> 
> Although I am having this issue on vhost_net,But it affects all users of
> unuse_mm.

I am confused. How can we ever assign the owner to a kernel thread. We
skip those explicitly. It simply doesn't make any sense to have an owner
a kernel thread.
-- 
Michal Hocko
SUSE Labs