xfrm: make local error reporting more robust

In xfrm4 and xfrm6 we need to take care about sockets of the other
address family. This could happen because a 6in4 or 4in6 tunnel could
get protected by ipsec.

Because we don't want to have a run-time dependency on ipv6 when only
using ipv4 xfrm we have to embed a pointer to the correct local_error
function in xfrm_state_afinet and look it up when returning an error
depending on the socket address family.

Thanks to vi0ss for the great bug report:
<https://bugzilla.kernel.org/show_bug.cgi?id=58691>

v2:
a) fix two more unsafe interpretations of skb->sk as ipv6 socket
   (xfrm6_local_dontfrag and __xfrm6_output)
v3:
a) add an EXPORT_SYMBOL_GPL(xfrm_local_error) to fix a link error when
   building ipv6 as a module (thanks to Steffen Klassert)

Reported-by: <vi0oss@gmail.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

2 files changed, 11 insertions, 2 deletions

diff --git a/net/ipv4/xfrm4_output.c b/net/ipv4/xfrm4_output.c
index 327a617d594c..7a5491ffa4de 100644
--- a/net/ipv4/xfrm4_output.c
+++ b/net/ipv4/xfrm4_output.c
@@ -33,8 +33,7 @@ static int xfrm4_tunnel_check_size(struct sk_buff *skb)
         mtu = dst_mtu(dst);
         if (skb->len > mtu) {
                 if (skb->sk)
-                        ip_local_error(skb->sk, EMSGSIZE, ip_hdr(skb)->daddr,
-                                       inet_sk(skb->sk)->inet_dport, mtu);
+                        xfrm_local_error(skb, mtu);
                 else
                         icmp_send(skb, ICMP_DEST_UNREACH,
                                   ICMP_FRAG_NEEDED, htonl(mtu));
@@ -99,3 +98,12 @@ int xfrm4_output(struct sk_buff *skb)
                             x->outer_mode->afinfo->output_finish,
                             !(IPCB(skb)->flags & IPSKB_REROUTED));
 }
+
+void xfrm4_local_error(struct sk_buff *skb, u32 mtu)
+{
+        struct iphdr *hdr;
+
+        hdr = skb->encapsulation ? inner_ip_hdr(skb) : ip_hdr(skb);
+        ip_local_error(skb->sk, EMSGSIZE, hdr->daddr,
+                       inet_sk(skb->sk)->inet_dport, mtu);
+}
diff --git a/net/ipv4/xfrm4_state.c b/net/ipv4/xfrm4_state.c
index 9258e751baba..0b2a0641526a 100644
--- a/net/ipv4/xfrm4_state.c
+++ b/net/ipv4/xfrm4_state.c
@@ -83,6 +83,7 @@ static struct xfrm_state_afinfo xfrm4_state_afinfo = {
         .extract_input          = xfrm4_extract_input,
         .extract_output         = xfrm4_extract_output,
         .transport_finish       = xfrm4_transport_finish,
+        .local_error            = xfrm4_local_error,
 };
 
 void __init xfrm4_state_init(void)