Merge branch 'l2tp-mtu'

Guillaume Nault says:

====================
l2tp: sanitise MTU handling on sessions

Most of the code handling sessions' MTU has no effect. The ->mtu field
in struct l2tp_session might be used at session creation time, but
neither PPP nor Ethernet pseudo-wires take updates into account.

L2TP sessions don't have a concept of MTU, which is the reason why
->mtu is mostly ignored. MTU should remain a network device thing.
Therefore this patch set does not try to propagate/update ->mtu to/from
the device. That would complicate the code unnecessarily. Instead this
field and the associated ioctl commands and netlink attributes are
removed.

Patch #1 defines l2tp_tunnel_dst_mtu() in order to simplify the
following patches. Then patches #2 and #3 remove MTU handling from PPP
and Ethernet pseudo-wires respectively.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>

7 files changed, 47 insertions, 87 deletions

diff --git a/include/uapi/linux/l2tp.h b/include/uapi/linux/l2tp.h
index 8bb8c7cfabe5..61158f5a1a5b 100644
--- a/include/uapi/linux/l2tp.h
+++ b/include/uapi/linux/l2tp.h
@@ -119,7 +119,7 @@ enum {
         L2TP_ATTR_IP_DADDR,             /* u32 */
         L2TP_ATTR_UDP_SPORT,            /* u16 */
         L2TP_ATTR_UDP_DPORT,            /* u16 */
-        L2TP_ATTR_MTU,                  /* u16 */
+        L2TP_ATTR_MTU,                  /* u16 (not used) */
         L2TP_ATTR_MRU,                  /* u16 (not used) */
         L2TP_ATTR_STATS,                /* nested */
         L2TP_ATTR_IP6_SADDR,            /* struct in6_addr */
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index c61a467fd9b8..ac6a00bcec71 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1674,7 +1674,6 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn
                 if (cfg) {
                         session->pwtype = cfg->pw_type;
                         session->debug = cfg->debug;
-                        session->mtu = cfg->mtu;
                         session->send_seq = cfg->send_seq;
                         session->recv_seq = cfg->recv_seq;
                         session->lns_mode = cfg->lns_mode;
diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index fa5ae9432d38..5804065dfbfb 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -12,6 +12,9 @@
 #ifndef _L2TP_CORE_H_
 #define _L2TP_CORE_H_
 
+#include <net/dst.h>
+#include <net/sock.h>
+
 /* Just some random numbers */
 #define L2TP_TUNNEL_MAGIC       0x42114DDA
 #define L2TP_SESSION_MAGIC      0x0C04EB7D
@@ -61,7 +64,6 @@ struct l2tp_session_cfg {
         int                     peer_cookie_len; /* 0, 4 or 8 bytes */
         int                     reorder_timeout; /* configured reorder timeout
                                                   * (in jiffies) */
-        int                     mtu;
         char                    *ifname;
 };
 
@@ -105,7 +107,6 @@ struct l2tp_session {
         int                     reorder_timeout; /* configured reorder timeout
                                                   * (in jiffies) */
         int                     reorder_skip;   /* set if skip to next nr */
-        int                     mtu;
         enum l2tp_pwtype        pwtype;
         struct l2tp_stats       stats;
         struct hlist_node       global_hlist;   /* Global hash list node */
@@ -268,6 +269,21 @@ static inline int l2tp_get_l2specific_len(struct l2tp_session *session)
         }
 }
 
+static inline u32 l2tp_tunnel_dst_mtu(const struct l2tp_tunnel *tunnel)
+{
+        struct dst_entry *dst;
+        u32 mtu;
+
+        dst = sk_dst_get(tunnel->sock);
+        if (!dst)
+                return 0;
+
+        mtu = dst_mtu(dst);
+        dst_release(dst);
+
+        return mtu;
+}
+
 #define l2tp_printk(ptr, type, func, fmt, ...)                          \
 do {                                                                    \
         if (((ptr)->debug) & (type))                                    \
diff --git a/net/l2tp/l2tp_debugfs.c b/net/l2tp/l2tp_debugfs.c
index aee271741f5b..9821a1458555 100644
--- a/net/l2tp/l2tp_debugfs.c
+++ b/net/l2tp/l2tp_debugfs.c
@@ -191,8 +191,7 @@ static void l2tp_dfs_seq_session_show(struct seq_file *m, void *v)
         if (session->send_seq || session->recv_seq)
                 seq_printf(m, "   nr %hu, ns %hu\n", session->nr, session->ns);
         seq_printf(m, "   refcnt %d\n", refcount_read(&session->ref_count));
-        seq_printf(m, "   config %d/0/%c/%c/-/%s %08x %u\n",
-                   session->mtu,
+        seq_printf(m, "   config 0/0/%c/%c/-/%s %08x %u\n",
                    session->recv_seq ? 'R' : '-',
                    session->send_seq ? 'S' : '-',
                    session->lns_mode ? "LNS" : "LAC",
diff --git a/net/l2tp/l2tp_eth.c b/net/l2tp/l2tp_eth.c
index 5c366ecfa1cb..3728986ec885 100644
--- a/net/l2tp/l2tp_eth.c
+++ b/net/l2tp/l2tp_eth.c
@@ -226,22 +226,19 @@ static void l2tp_eth_adjust_mtu(struct l2tp_tunnel *tunnel,
                                 struct net_device *dev)
 {
         unsigned int overhead = 0;
-        struct dst_entry *dst;
         u32 l3_overhead = 0;
+        u32 mtu;
 
         /* if the encap is UDP, account for UDP header size */
         if (tunnel->encap == L2TP_ENCAPTYPE_UDP) {
                 overhead += sizeof(struct udphdr);
                 dev->needed_headroom += sizeof(struct udphdr);
         }
-        if (session->mtu != 0) {
-                dev->mtu = session->mtu;
-                dev->needed_headroom += session->hdr_len;
-                return;
-        }
+
         lock_sock(tunnel->sock);
         l3_overhead = kernel_sock_ip_overhead(tunnel->sock);
         release_sock(tunnel->sock);
+
         if (l3_overhead == 0) {
                 /* L3 Overhead couldn't be identified, this could be
                  * because tunnel->sock was NULL or the socket's
@@ -255,18 +252,12 @@ static void l2tp_eth_adjust_mtu(struct l2tp_tunnel *tunnel,
          */
         overhead += session->hdr_len + ETH_HLEN + l3_overhead;
 
-        /* If PMTU discovery was enabled, use discovered MTU on L2TP device */
-        dst = sk_dst_get(tunnel->sock);
-        if (dst) {
-                /* dst_mtu will use PMTU if found, else fallback to intf MTU */
-                u32 pmtu = dst_mtu(dst);
+        mtu = l2tp_tunnel_dst_mtu(tunnel) - overhead;
+        if (mtu < dev->min_mtu || mtu > dev->max_mtu)
+                dev->mtu = ETH_DATA_LEN - overhead;
+        else
+                dev->mtu = mtu;
 
-                if (pmtu != 0)
-                        dev->mtu = pmtu;
-                dst_release(dst);
-        }
-        session->mtu = dev->mtu - overhead;
-        dev->mtu = session->mtu;
         dev->needed_headroom += session->hdr_len;
 }
 
diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index a7c409215336..2e1e92651545 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -608,9 +608,6 @@ static int l2tp_nl_cmd_session_create(struct sk_buff *skb, struct genl_info *inf
         if (info->attrs[L2TP_ATTR_RECV_TIMEOUT])
                 cfg.reorder_timeout = nla_get_msecs(info->attrs[L2TP_ATTR_RECV_TIMEOUT]);
 
-        if (info->attrs[L2TP_ATTR_MTU])
-                cfg.mtu = nla_get_u16(info->attrs[L2TP_ATTR_MTU]);
-
 #ifdef CONFIG_MODULES
         if (l2tp_nl_cmd_ops[cfg.pw_type] == NULL) {
                 genl_unlock();
@@ -698,9 +695,6 @@ static int l2tp_nl_cmd_session_modify(struct sk_buff *skb, struct genl_info *inf
         if (info->attrs[L2TP_ATTR_RECV_TIMEOUT])
                 session->reorder_timeout = nla_get_msecs(info->attrs[L2TP_ATTR_RECV_TIMEOUT]);
 
-        if (info->attrs[L2TP_ATTR_MTU])
-                session->mtu = nla_get_u16(info->attrs[L2TP_ATTR_MTU]);
-
         ret = l2tp_session_notify(&l2tp_nl_family, info,
                                   session, L2TP_CMD_SESSION_MODIFY);
 
@@ -730,8 +724,7 @@ static int l2tp_nl_session_send(struct sk_buff *skb, u32 portid, u32 seq, int fl
             nla_put_u32(skb, L2TP_ATTR_PEER_SESSION_ID,
                         session->peer_session_id) ||
             nla_put_u32(skb, L2TP_ATTR_DEBUG, session->debug) ||
-            nla_put_u16(skb, L2TP_ATTR_PW_TYPE, session->pwtype) ||
-            nla_put_u16(skb, L2TP_ATTR_MTU, session->mtu))
+            nla_put_u16(skb, L2TP_ATTR_PW_TYPE, session->pwtype))
                 goto nla_put_failure;
 
         if ((session->ifname[0] &&
diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 44cac66284a5..b403728e2757 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -93,7 +93,6 @@
 #include <linux/nsproxy.h>
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>
-#include <net/dst.h>
 #include <net/ip.h>
 #include <net/udp.h>
 #include <net/xfrm.h>
@@ -554,7 +553,6 @@ static void pppol2tp_show(struct seq_file *m, void *arg)
 static void pppol2tp_session_init(struct l2tp_session *session)
 {
         struct pppol2tp_session *ps;
-        struct dst_entry *dst;
 
         session->recv_skb = pppol2tp_recv;
 #if IS_ENABLED(CONFIG_L2TP_DEBUGFS)
@@ -564,17 +562,6 @@ static void pppol2tp_session_init(struct l2tp_session *session)
         ps = l2tp_session_priv(session);
         mutex_init(&ps->sk_lock);
         ps->owner = current->pid;
-
-        /* If PMTU discovery was enabled, use the MTU that was discovered */
-        dst = sk_dst_get(session->tunnel->sock);
-        if (dst) {
-                u32 pmtu = dst_mtu(dst);
-
-                if (pmtu)
-                        session->mtu = pmtu - PPPOL2TP_HEADER_OVERHEAD;
-
-                dst_release(dst);
-        }
 }
 
 struct l2tp_connect_info {
@@ -661,6 +648,22 @@ static int pppol2tp_sockaddr_get_info(const void *sa, int sa_len,
         return 0;
 }
 
+/* Rough estimation of the maximum payload size a tunnel can transmit without
+ * fragmenting at the lower IP layer. Assumes L2TPv2 with sequence
+ * numbers and no IP option. Not quite accurate, but the result is mostly
+ * unused anyway.
+ */
+static int pppol2tp_tunnel_mtu(const struct l2tp_tunnel *tunnel)
+{
+        int mtu;
+
+        mtu = l2tp_tunnel_dst_mtu(tunnel);
+        if (mtu <= PPPOL2TP_HEADER_OVERHEAD)
+                return 1500 - PPPOL2TP_HEADER_OVERHEAD;
+
+        return mtu - PPPOL2TP_HEADER_OVERHEAD;
+}
+
 /* connect() handler. Attach a PPPoX socket to a tunnel UDP socket
  */
 static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
@@ -778,8 +781,6 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
                         goto end;
                 }
         } else {
-                /* Default MTU must allow space for UDP/L2TP/PPP headers */
-                cfg.mtu = 1500 - PPPOL2TP_HEADER_OVERHEAD;
                 cfg.pw_type = L2TP_PWTYPE_PPP;
 
                 session = l2tp_session_create(sizeof(struct pppol2tp_session),
@@ -824,7 +825,7 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 
         po->chan.private = sk;
         po->chan.ops     = &pppol2tp_chan_ops;
-        po->chan.mtu     = session->mtu;
+        po->chan.mtu     = pppol2tp_tunnel_mtu(tunnel);
 
         error = ppp_register_net_channel(sock_net(sk), &po->chan);
         if (error) {
@@ -880,10 +881,6 @@ static int pppol2tp_session_create(struct net *net, struct l2tp_tunnel *tunnel,
                 goto err;
         }
 
-        /* Default MTU values. */
-        if (cfg->mtu == 0)
-                cfg->mtu = 1500 - PPPOL2TP_HEADER_OVERHEAD;
-
         /* Allocate and initialize a new session context. */
         session = l2tp_session_create(sizeof(struct pppol2tp_session),
                                       tunnel, session_id,
@@ -1047,7 +1044,6 @@ static void pppol2tp_copy_stats(struct pppol2tp_ioc_stats *dest,
 static int pppol2tp_session_ioctl(struct l2tp_session *session,
                                   unsigned int cmd, unsigned long arg)
 {
-        struct ifreq ifr;
         int err = 0;
         struct sock *sk;
         int val = (int) arg;
@@ -1063,39 +1059,6 @@ static int pppol2tp_session_ioctl(struct l2tp_session *session,
                 return -EBADR;
 
         switch (cmd) {
-        case SIOCGIFMTU:
-                err = -ENXIO;
-                if (!(sk->sk_state & PPPOX_CONNECTED))
-                        break;
-
-                err = -EFAULT;
-                if (copy_from_user(&ifr, (void __user *) arg, sizeof(struct ifreq)))
-                        break;
-                ifr.ifr_mtu = session->mtu;
-                if (copy_to_user((void __user *) arg, &ifr, sizeof(struct ifreq)))
-                        break;
-
-                l2tp_info(session, L2TP_MSG_CONTROL, "%s: get mtu=%d\n",
-                          session->name, session->mtu);
-                err = 0;
-                break;
-
-        case SIOCSIFMTU:
-                err = -ENXIO;
-                if (!(sk->sk_state & PPPOX_CONNECTED))
-                        break;
-
-                err = -EFAULT;
-                if (copy_from_user(&ifr, (void __user *) arg, sizeof(struct ifreq)))
-                        break;
-
-                session->mtu = ifr.ifr_mtu;
-
-                l2tp_info(session, L2TP_MSG_CONTROL, "%s: set mtu=%d\n",
-                          session->name, session->mtu);
-                err = 0;
-                break;
-
         case PPPIOCGMRU:
         case PPPIOCGFLAGS:
                 err = -EFAULT;
@@ -1692,8 +1655,7 @@ static void pppol2tp_seq_session_show(struct seq_file *m, void *v)
                    tunnel->peer_tunnel_id,
                    session->peer_session_id,
                    state, user_data_ok);
-        seq_printf(m, "   %d/0/%c/%c/%s %08x %u\n",
-                   session->mtu,
+        seq_printf(m, "   0/0/%c/%c/%s %08x %u\n",
                    session->recv_seq ? 'R' : '-',
                    session->send_seq ? 'S' : '-',
                    session->lns_mode ? "LNS" : "LAC",