l2tp: fix kernel panic when using IPv4-mapped IPv6 addresses

IPv4 mapped addresses cause kernel panic.
The patch juste check whether the IPv6 address is an IPv4 mapped
address. If so, use IPv4 API instead of IPv6.

[  940.026915] general protection fault: 0000 [#1]
[  940.026915] Modules linked in: l2tp_ppp l2tp_netlink l2tp_core pppox ppp_generic slhc loop psmouse
[  940.026915] CPU: 0 PID: 3184 Comm: memcheck-amd64- Not tainted 3.11.0+ #1
[  940.026915] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
[  940.026915] task: ffff880007130e20 ti: ffff88000737e000 task.ti: ffff88000737e000
[  940.026915] RIP: 0010:[<ffffffff81333780>]  [<ffffffff81333780>] ip6_xmit+0x276/0x326
[  940.026915] RSP: 0018:ffff88000737fd28  EFLAGS: 00010286
[  940.026915] RAX: c748521a75ceff48 RBX: ffff880000c30800 RCX: 0000000000000000
[  940.026915] RDX: ffff88000075cc4e RSI: 0000000000000028 RDI: ffff8800060e5a40
[  940.026915] RBP: ffff8800060e5a40 R08: 0000000000000000 R09: ffff88000075cc90
[  940.026915] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88000737fda0
[  940.026915] R13: 0000000000000000 R14: 0000000000002000 R15: ffff880005d3b580
[  940.026915] FS:  00007f163dc5e800(0000) GS:ffffffff81623000(0000) knlGS:0000000000000000
[  940.026915] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  940.026915] CR2: 00000004032dc940 CR3: 0000000005c25000 CR4: 00000000000006f0
[  940.026915] Stack:
[  940.026915]  ffff88000075cc4e ffffffff81694e90 ffff880000c30b38 0000000000000020
[  940.026915]  11000000523c4bac ffff88000737fdb4 0000000000000000 ffff880000c30800
[  940.026915]  ffff880005d3b580 ffff880000c30b38 ffff8800060e5a40 0000000000000020
[  940.026915] Call Trace:
[  940.026915]  [<ffffffff81356cc3>] ? inet6_csk_xmit+0xa4/0xc4
[  940.026915]  [<ffffffffa0038535>] ? l2tp_xmit_skb+0x503/0x55a [l2tp_core]
[  940.026915]  [<ffffffff812b8d3b>] ? pskb_expand_head+0x161/0x214
[  940.026915]  [<ffffffffa003e91d>] ? pppol2tp_xmit+0xf2/0x143 [l2tp_ppp]
[  940.026915]  [<ffffffffa00292e0>] ? ppp_channel_push+0x36/0x8b [ppp_generic]
[  940.026915]  [<ffffffffa00293fe>] ? ppp_write+0xaf/0xc5 [ppp_generic]
[  940.026915]  [<ffffffff8110ead4>] ? vfs_write+0xa2/0x106
[  940.026915]  [<ffffffff8110edd6>] ? SyS_write+0x56/0x8a
[  940.026915]  [<ffffffff81378ac0>] ? system_call_fastpath+0x16/0x1b
[  940.026915] Code: 00 49 8b 8f d8 00 00 00 66 83 7c 11 02 00 74 60 49
8b 47 58 48 83 e0 fe 48 8b 80 18 01 00 00 48 85 c0 74 13 48 8b 80 78 02
00 00 <48> ff 40 28 41 8b 57 68 48 01 50 30 48 8b 54 24 08 49 c7 c1 51
[  940.026915] RIP  [<ffffffff81333780>] ip6_xmit+0x276/0x326
[  940.026915]  RSP <ffff88000737fd28>
[  940.057945] ---[ end trace be8aba9a61c8b7f3 ]---
[  940.058583] Kernel panic - not syncing: Fatal exception in interrupt

Signed-off-by: François CACHEREUL <f.cachereul@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 26 insertions, 4 deletions

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index feae495a0a30..aedaa2cd4237 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -496,6 +496,7 @@ out:
 static inline int l2tp_verify_udp_checksum(struct sock *sk,
                                            struct sk_buff *skb)
 {
+        struct l2tp_tunnel *tunnel = (struct l2tp_tunnel *)sk->sk_user_data;
         struct udphdr *uh = udp_hdr(skb);
         u16 ulen = ntohs(uh->len);
         __wsum psum;
@@ -504,7 +505,7 @@ static inline int l2tp_verify_udp_checksum(struct sock *sk,
                 return 0;
 
 #if IS_ENABLED(CONFIG_IPV6)
-        if (sk->sk_family == PF_INET6) {
+        if (sk->sk_family == PF_INET6 && !tunnel->v4mapped) {
                 if (!uh->check) {
                         LIMIT_NETDEBUG(KERN_INFO "L2TP: IPv6: checksum is 0\n");
                         return 1;
@@ -1128,7 +1129,7 @@ static int l2tp_xmit_core(struct l2tp_session *session, struct sk_buff *skb,
         /* Queue the packet to IP for output */
         skb->local_df = 1;
 #if IS_ENABLED(CONFIG_IPV6)
-        if (skb->sk->sk_family == PF_INET6)
+        if (skb->sk->sk_family == PF_INET6 && !tunnel->v4mapped)
                 error = inet6_csk_xmit(skb, NULL);
         else
 #endif
@@ -1255,7 +1256,7 @@ int l2tp_xmit_skb(struct l2tp_session *session, struct sk_buff *skb, int hdr_len
 
                 /* Calculate UDP checksum if configured to do so */
 #if IS_ENABLED(CONFIG_IPV6)
-                if (sk->sk_family == PF_INET6)
+                if (sk->sk_family == PF_INET6 && !tunnel->v4mapped)
                         l2tp_xmit_ipv6_csum(sk, skb, udp_len);
                 else
 #endif
@@ -1704,6 +1705,24 @@ int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id, u32
         if (cfg != NULL)
                 tunnel->debug = cfg->debug;
 
+#if IS_ENABLED(CONFIG_IPV6)
+        if (sk->sk_family == PF_INET6) {
+                struct ipv6_pinfo *np = inet6_sk(sk);
+
+                if (ipv6_addr_v4mapped(&np->saddr) &&
+                    ipv6_addr_v4mapped(&np->daddr)) {
+                        struct inet_sock *inet = inet_sk(sk);
+
+                        tunnel->v4mapped = true;
+                        inet->inet_saddr = np->saddr.s6_addr32[3];
+                        inet->inet_rcv_saddr = np->rcv_saddr.s6_addr32[3];
+                        inet->inet_daddr = np->daddr.s6_addr32[3];
+                } else {
+                        tunnel->v4mapped = false;
+                }
+        }
+#endif
+
         /* Mark socket as an encapsulation socket. See net/ipv4/udp.c */
         tunnel->encap = encap;
         if (encap == L2TP_ENCAPTYPE_UDP) {
@@ -1712,7 +1731,7 @@ int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id, u32
                 udp_sk(sk)->encap_rcv = l2tp_udp_encap_recv;
                 udp_sk(sk)->encap_destroy = l2tp_udp_encap_destroy;
 #if IS_ENABLED(CONFIG_IPV6)
-                if (sk->sk_family == PF_INET6)
+                if (sk->sk_family == PF_INET6 && !tunnel->v4mapped)
                         udpv6_encap_enable();
                 else
 #endif
diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index 66a559b104b6..6f251cbc2ed7 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -194,6 +194,9 @@ struct l2tp_tunnel {
         struct sock             *sock;          /* Parent socket */
         int                     fd;             /* Parent fd, if tunnel socket
                                                  * was created by userspace */
+#if IS_ENABLED(CONFIG_IPV6)
+        bool                    v4mapped;
+#endif
 
         struct work_struct      del_work;