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Date:   Sun, 2 Aug 2020 19:22:26 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Rustam Kovhaev <rkovhaev@gmail.com>
Cc:     dledford@redhat.com, linux-kernel@vger.kernel.org,
        linux-rdma@vger.kernel.org, syzkaller-bugs@googlegroups.com
Subject: Re: KASAN: use-after-free Read in netdevice_event_work_handler
Message-ID: <20200802222226.GO24045@ziepe.ca>
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 <20200731211122.GA1728751@thinkpad>
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On Fri, Jul 31, 2020 at 02:11:22PM -0700, Rustam Kovhaev wrote:

> IB roce driver receives NETDEV_UNREGISTER event, calls dev_hold() and
> schedules work item to execute, and before wq gets a chance to complete
> it, we return to ip_tunnel.c:274 and call free_netdev(), and then later
> we get UAF when scheduled function references already freed net_device
> 
> i added verbose logging to ip_tunnel.c to see pcpu_refcnt:
> +       pr_info("about to free_netdev(dev) dev->pcpu_refcnt %d", netdev_refcnt_read(dev));
> 
> and got the following:
> [  410.220127][ T2944] ip_tunnel: about to free_netdev(dev) dev->pcpu_refcnt 8

I think there is a missing call to netdev_wait_allrefs() in the
rollback_registered_many().

The normal success flow has this wait after delivering
NETDEV_UNREGISTER, the error unwind for register_netdevice should as
well.

If the netdevice can progress to free while a dev_hold is active I
think it means dev_hold is functionally useless.

Jason