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SubjectRe: [PATCH v1 2/2] hamradio: defer 6pack kfree after unregister_netdev
On Mon,  8 Nov 2021 18:37:59 +0800 Lin Ma wrote:
> There is a possible race condition (use-after-free) like below
>
> (USE) | (FREE)
> dev_queue_xmit |
> __dev_queue_xmit |
> __dev_xmit_skb |
> sch_direct_xmit | ...
> xmit_one |
> netdev_start_xmit | tty_ldisc_kill
> __netdev_start_xmit | 6pack_close
> sp_xmit | kfree
> sp_encaps |
> |
>
> According to the patch "defer ax25 kfree after unregister_netdev", this

This is the previous patch of the series? Maybe call it "previous
patch"?

> patch reorder the kfree after the unregister_netdev to avoid the possible
> UAF as the unregister_netdev() is well synchronized and won't return if
> there is a running routine.
>
> Signed-off-by: Lin Ma <linma@zju.edu.cn>

> diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
> index 49f10053a794..bfdf89e54752 100644
> --- a/drivers/net/hamradio/6pack.c
> +++ b/drivers/net/hamradio/6pack.c
> @@ -672,11 +672,13 @@ static void sixpack_close(struct tty_struct *tty)
> del_timer_sync(&sp->tx_t);
> del_timer_sync(&sp->resync_t);
>
> - /* Free all 6pack frame buffers. */
> + unregister_netdev(sp->dev);
> +
> + /* Free all 6pack frame buffers after unreg. */
> kfree(sp->rbuff);
> kfree(sp->xbuff);
>
> - unregister_netdev(sp->dev);
> + free_netdev(sp->dev);

You should mention in the commit message why you think it's safe to add
free_netdev() which wasn't here before...

This driver seems to be setting:

dev->needs_free_netdev = true;

so unregister_netdev() will free the netdev automatically.

That said I don't see a reason why this driver needs the automatic
cleanup.

You can either remove that setting and then you can call free_netdev()
like you do, or you need to move the cleanup to dev->priv_destructor.

> }
>
> /* Perform I/O control on an active 6pack channel. */

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