On Fri, Dec 04, 2020 at 04:36:55PM +0000, Tom Parkin wrote:
> +static int ppp_unbridge_channels(struct channel *pch)
> +{
> + struct channel *pchb, *pchbb;
> +
> + write_lock_bh(&pch->upl);
> + pchb = rcu_dereference_protected(pch->bridge,
> lockdep_is_held(&pch->upl));
> + if (!pchb) {
> + write_unlock_bh(&pch->upl);
> + return -EINVAL;
> + }
> + RCU_INIT_POINTER(pch->bridge, NULL);
> + write_unlock_bh(&pch->upl);
> +
> + write_lock_bh(&pchb->upl);
> + pchbb = rcu_dereference_protected(pchb->bridge,
> lockdep_is_held(&pchb->upl));
> + if (pchbb == pch)
> + RCU_INIT_POINTER(pchb->bridge, NULL);
> + write_unlock_bh(&pchb->upl);
> +
> + synchronize_rcu();
> +
> + if (pchbb == pch)
> + if (refcount_dec_and_test(&pch->file.refcnt))
> + ppp_destroy_channel(pch);
Since a respin is needed (see below), maybe add a comment explaining
why we need to verify that pchbb == pch.
> + if (refcount_dec_and_test(&pchb->file.refcnt))
> + ppp_destroy_channel(pchb);
> +
> + return 0;
> +}
> +
> static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> {
> struct ppp_file *pf;
> @@ -641,8 +714,9 @@ static long ppp_ioctl(struct file *file, unsigned int
> cmd, unsigned long arg)
> }
>
> if (pf->kind == CHANNEL) {
> - struct channel *pch;
> + struct channel *pch, *pchb;
> struct ppp_channel *chan;
> + struct ppp_net *pn;
>
> pch = PF_TO_CHANNEL(pf);
>
> @@ -657,6 +731,29 @@ static long ppp_ioctl(struct file *file, unsigned int
> cmd, unsigned long arg)
> err = ppp_disconnect_channel(pch);
> break;
>
> + case PPPIOCBRIDGECHAN:
> + if (get_user(unit, p))
> + break;
> + err = -ENXIO;
> + pn = ppp_pernet(current->nsproxy->net_ns);
> + spin_lock_bh(&pn->all_channels_lock);
> + pchb = ppp_find_channel(pn, unit);
> + /* Hold a reference to prevent pchb being freed while
> + * we establish the bridge.
> + */
> + if (pchb)
> + refcount_inc(&pchb->file.refcnt);
The !pchb case isn't handled. With this code, if ppp_find_channel()
returns NULL, ppp_bridge_channels() will crash when trying to lock
pchb->upl.
> + spin_unlock_bh(&pn->all_channels_lock);
> + err = ppp_bridge_channels(pch, pchb);
> + /* Drop earlier refcount now bridge establishment is
> complete */
> + if (refcount_dec_and_test(&pchb->file.refcnt))
> + ppp_destroy_channel(pchb);
> + break;
> +
The rest looks good to me.
