Date:   Thu, 26 Nov 2020 12:24:25 +0000
From:   Tom Parkin <tparkin@...alix.com>
To:     netdev@...r.kernel.org
Cc:     gnault@...hat.com, jchapman@...alix.com,
        Tom Parkin <tparkin@...alix.com>
Subject: [PATCH net-next 1/2] ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls

This new ioctl pair allows two ppp channels to be bridged together:
frames arriving in one channel are transmitted in the other channel
and vice versa.

The practical use for this is primarily to support the L2TP Access
Concentrator use-case.  The end-user session is presented as a ppp
channel (typically PPPoE, although it could be e.g. PPPoA, or even PPP
over a serial link) and is switched into a PPPoL2TP session for
transmission to the LNS.  At the LNS the PPP session is terminated in
the ISP's network.

When a PPP channel is bridged to another it takes a reference on the
other's struct ppp_file.  This reference is dropped when the channels
are unbridged, which can occur either explicitly on userspace calling
the PPPIOCUNBRIDGECHAN ioctl, or implicitly when either channel in the
bridge is unregistered.

In order to implement the channel bridge, struct channel is extended
with a new field, 'bridge', which points to the other struct channel
making up the bridge.

This pointer is RCU protected to avoid adding another lock to the data
path.

To guard against concurrent writes to the pointer, the existing struct
channel lock 'downl' use is extended (rather than adding a new lock).
Order of lock acquisition is maintained: i.e. the channel 'upl' lock is
always acquired before 'downl' in code paths that need to hold both.

Signed-off-by: Tom Parkin <tparkin@...alix.com>
---
 drivers/net/ppp/ppp_generic.c  | 147 ++++++++++++++++++++++++++++++++-
 include/uapi/linux/ppp-ioctl.h |   2 +
 2 files changed, 147 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 7d005896a0f9..5e563bfb8e2a 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -170,11 +170,12 @@ struct channel {
 	struct list_head list;		/* link in all/new_channels list */
 	struct ppp_channel *chan;	/* public channel data structure */
 	struct rw_semaphore chan_sem;	/* protects `chan' during chan ioctl */
-	spinlock_t	downl;		/* protects `chan', file.xq dequeue */
+	spinlock_t	downl;		/* protects `chan', 'bridge', file.xq dequeue */
 	struct ppp	*ppp;		/* ppp unit we're connected to */
 	struct net	*chan_net;	/* the net channel belongs to */
 	struct list_head clist;		/* link in list of channels per unit */
 	rwlock_t	upl;		/* protects `ppp' */
+	struct channel *bridge;		/* "bridged" ppp channel */
 #ifdef CONFIG_PPP_MULTILINK
 	u8		avail;		/* flag used in multilink stuff */
 	u8		had_frag;	/* >= 1 fragments have been sent */
@@ -606,6 +607,95 @@ static struct bpf_prog *compat_ppp_get_filter(struct sock_fprog32 __user *p)
 #endif
 #endif
 
+/* Bridge one PPP channel to another.
+ * When two channels are bridged, ppp_input on one channel is redirected to
+ * the other's ops->start_xmit handler.
+ * In order to safely bridge channels we must reject channels which are already
+ * part of a bridge instance, or which form part of an existing unit.
+ * Once successfully bridged, each channel holds a reference on the other
+ * to prevent it being freed while the bridge is extant.
+ */
+static int ppp_bridge_channels(struct channel *pch, struct channel *pchb)
+{
+	int ret = -EALREADY;
+
+	/* We need to take each channel upl for access to the 'ppp' field,
+	 * and each channel downl for write access to the 'bridge' field.
+	 */
+
+	read_lock_bh(&pch->upl);
+	if (pch->ppp)
+		goto out0;
+
+	spin_lock(&pch->downl);
+
+	read_lock_bh(&pchb->upl);
+	if (pchb->ppp)
+		goto out1;
+
+	spin_lock(&pchb->downl);
+
+	if (pch->bridge || pchb->bridge)
+		goto out2;
+
+	rcu_assign_pointer(pch->bridge, pchb);
+	refcount_inc(&pchb->file.refcnt);
+
+	rcu_assign_pointer(pchb->bridge, pch);
+	refcount_inc(&pch->file.refcnt);
+
+	ret = 0;
+
+out2:
+	spin_unlock(&pchb->downl);
+out1:
+	read_unlock_bh(&pchb->upl);
+	spin_unlock(&pch->downl);
+out0:
+	read_unlock_bh(&pch->upl);
+
+	return ret;
+}
+
+static int ppp_unbridge_channels(struct channel *pch)
+{
+	struct channel *pchb;
+
+	rcu_read_lock();
+
+	pchb = rcu_dereference(pch->bridge);
+	if (!pchb) {
+		rcu_read_unlock();
+		return -ENXIO;
+	}
+
+	if (pch != rcu_dereference(pchb->bridge)) {
+		rcu_read_unlock();
+		return -ENXIO;
+	}
+
+	spin_lock(&pch->downl);
+	spin_lock(&pchb->downl);
+
+	rcu_assign_pointer(pch->bridge, NULL);
+	rcu_assign_pointer(pchb->bridge, NULL);
+
+	spin_unlock(&pchb->downl);
+	spin_unlock(&pch->downl);
+
+	rcu_read_unlock();
+
+	synchronize_rcu();
+
+	if (refcount_dec_and_test(&pch->file.refcnt))
+		ppp_destroy_channel(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 +731,9 @@ 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 +748,22 @@ 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);
+			if (pchb)
+				err = ppp_bridge_channels(pch, pchb);
+			spin_unlock_bh(&pn->all_channels_lock);
+			break;
+
+		case PPPIOCUNBRIDGECHAN:
+			err = ppp_unbridge_channels(pch);
+			break;
+
 		default:
 			down_read(&pch->chan_sem);
 			chan = pch->chan;
@@ -2089,6 +2196,35 @@ static bool ppp_decompress_proto(struct sk_buff *skb)
 	return pskb_may_pull(skb, 2);
 }
 
+/* Attempt to handle a frame via. a bridged channel, if one exists.
+ * If the channel is bridged, the frame is consumed by the bridge.
+ * If not, the caller must handle the frame by normal recv mechanisms.
+ * Returns true if the frame is consumed, false otherwise.
+ */
+static bool ppp_channel_bridge_input(struct channel *pch, struct sk_buff *skb)
+{
+	struct channel *pchb;
+
+	rcu_read_lock();
+	pchb = rcu_dereference(pch->bridge);
+	if (pchb) {
+		spin_lock(&pchb->downl);
+		if (pchb->chan) {
+			skb_scrub_packet(skb, !net_eq(pch->chan_net, pchb->chan_net));
+			if (!pchb->chan->ops->start_xmit(pchb->chan, skb))
+				kfree_skb(skb);
+		} else {
+			/* channel got unregistered */
+			kfree_skb(skb);
+		}
+		spin_unlock(&pchb->downl);
+	}
+	rcu_read_unlock();
+
+	/* If pchb is set then we've consumed the packet */
+	return pchb;
+}
+
 void
 ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
 {
@@ -2100,6 +2236,10 @@ ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
 		return;
 	}
 
+	/* If the channel is bridged, transmit via. bridge */
+	if (ppp_channel_bridge_input(pch, skb))
+		return;
+
 	read_lock_bh(&pch->upl);
 	if (!ppp_decompress_proto(skb)) {
 		kfree_skb(skb);
@@ -2796,8 +2936,11 @@ ppp_unregister_channel(struct ppp_channel *chan)
 	list_del(&pch->list);
 	spin_unlock_bh(&pn->all_channels_lock);
 
+	ppp_unbridge_channels(pch);
+
 	pch->file.dead = 1;
 	wake_up_interruptible(&pch->file.rwait);
+
 	if (refcount_dec_and_test(&pch->file.refcnt))
 		ppp_destroy_channel(pch);
 }
diff --git a/include/uapi/linux/ppp-ioctl.h b/include/uapi/linux/ppp-ioctl.h
index 7bd2a5a75348..8dbecb3ad036 100644
--- a/include/uapi/linux/ppp-ioctl.h
+++ b/include/uapi/linux/ppp-ioctl.h
@@ -115,6 +115,8 @@ struct pppol2tp_ioc_stats {
 #define PPPIOCATTCHAN	_IOW('t', 56, int)	/* attach to ppp channel */
 #define PPPIOCGCHAN	_IOR('t', 55, int)	/* get ppp channel number */
 #define PPPIOCGL2TPSTATS _IOR('t', 54, struct pppol2tp_ioc_stats)
+#define PPPIOCBRIDGECHAN _IOW('t', 53, int)	/* bridge one channel to another */
+#define PPPIOCUNBRIDGECHAN _IO('t', 54)	/* unbridge channel */
 
 #define SIOCGPPPSTATS   (SIOCDEVPRIVATE + 0)
 #define SIOCGPPPVER     (SIOCDEVPRIVATE + 1)	/* NEVER change this!! */
-- 
2.17.1

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