inet: add RCU protection to inet->opt

We lack proper synchronization to manipulate inet->opt ip_options

Problem is ip_make_skb() calls ip_setup_cork() and
ip_setup_cork() possibly makes a copy of ipc->opt (struct ip_options),
without any protection against another thread manipulating inet->opt.

Another thread can change inet->opt pointer and free old one under us.

Use RCU to protect inet->opt (changed to inet->inet_opt).

Instead of handling atomic refcounts, just copy ip_options when
necessary, to avoid cache line dirtying.

We cant insert an rcu_head in struct ip_options since its included in
skb->cb[], so this patch is large because I had to introduce a new
ip_options_rcu structure.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 3 deletions

diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index cc673677c5de..962a607b51da 100644
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -416,7 +416,6 @@ static int l2tp_ip_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *m
         int rc;
         struct l2tp_ip_sock *lsa = l2tp_ip_sk(sk);
         struct inet_sock *inet = inet_sk(sk);
-        struct ip_options *opt = inet->opt;
         struct rtable *rt = NULL;
         int connected = 0;
         __be32 daddr;
@@ -471,9 +470,14 @@ static int l2tp_ip_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *m
                 rt = (struct rtable *) __sk_dst_check(sk, 0);
 
         if (rt == NULL) {
+                struct ip_options_rcu *inet_opt;
+
+                inet_opt = rcu_dereference_protected(inet->inet_opt,
+                                                     sock_owned_by_user(sk));
+
                 /* Use correct destination address if we have options. */
-                if (opt && opt->srr)
-                        daddr = opt->faddr;
+                if (inet_opt && inet_opt->opt.srr)
+                        daddr = inet_opt->opt.faddr;
 
                 /* If this fails, retransmit mechanism of transport layer will
                  * keep trying until route appears or the connection times