Re: [PATCH net v2] nfc: digital: fix use-after-free in nfc_digital_unregister_device()

[Simon Horman <horms@kernel.org>]

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Subject: nfc: digital: fix use-after-free in nfc_digital_unregister_device()

This patch replaces cancel_work_sync() with disable_work_sync() on
cmd_work and cmd_complete_work in nfc_digital_unregister_device() so
that re-arming schedule_work() calls become no-ops before the cmd_queue
is freed.

> diff --git a/net/nfc/digital_core.c b/net/nfc/digital_core.c
> index 7cb1e6aaae90..6def5132a4a6 100644
> --- a/net/nfc/digital_core.c
> +++ b/net/nfc/digital_core.c
> @@ -843,8 +843,8 @@ void nfc_digital_unregister_device(struct nfc_digital_dev *ddev)
>  	mutex_unlock(&ddev->poll_lock);
>  
>  	cancel_delayed_work_sync(&ddev->poll_work);
> -	cancel_work_sync(&ddev->cmd_work);
> -	cancel_work_sync(&ddev->cmd_complete_work);
> +	disable_work_sync(&ddev->cmd_work);
> +	disable_work_sync(&ddev->cmd_complete_work);

[High]
Does this fully close the use-after-free window, or only the workqueue
re-arm portion of it?

Once digital_wq_cmd() has handed a command to the driver via
ops->in_send_cmd(..., digital_send_cmd_complete, cmd), the driver
retains a raw pointer to that digital_cmd in its own async machinery
(for nfcsim this is dev->recv_work / dev->arg, for real drivers it is
URB completions, timers, etc.).

disable_work_sync() on cmd_work and cmd_complete_work waits only for
those two work items; it does not synchronize the driver's async
completion path.  When that completion later fires it calls back into
digital_send_cmd_complete():

    static void digital_send_cmd_complete(struct nfc_digital_dev *ddev,
                                          void *arg, struct sk_buff *resp)
    {
            struct digital_cmd *cmd = arg;

            cmd->resp = resp;

            schedule_work(&ddev->cmd_complete_work);
    }

The subsequent schedule_work() is correctly a no-op after
disable_work_sync(), but the cmd->resp = resp store happens first, on
a digital_cmd that the cleanup loop in nfc_digital_unregister_device()
may already have kfree()'d.

The same reproducer described in the commit message applies: in
nfcsim_device_free(), nfc_digital_unregister_device() runs before
cancel_work_sync(&dev->recv_work) and before dev->up = false, so a
pending dev->recv_work blocked in wait_event can wake up after the
queue has been freed and call dev->cb(ddev, dev->arg, skb) with
dev->arg pointing at a freed digital_cmd.

Elsewhere in this file the pattern for stopping in-flight commands
before touching the queue calls digital_abort_cmd(ddev) (see
digital_stop_poll(), digital_dep_link_down(), and
digital_deactivate_target()), which asks the driver to abort its
outstanding commands.  Would an equivalent digital_abort_cmd(ddev) in
nfc_digital_unregister_device(), before the list_for_each_entry_safe()
kfree loop, be needed here as well?

The commit message states:

    Once both works are disabled no work can run, and the cleanup
    loop frees the queue with no work able to reach a freed command.

Is that description accurate given that the driver's completion
callback reaches the digital_cmd directly through the arg pointer
rather than through cmd_work or cmd_complete_work?

This residual issue is pre-existing rather than introduced by the
patch, but it seems relevant to the completeness of the fix as
described.