ipv6: kill sk_dst_lock

While testing the np->opt RCU conversion, I found that UDP/IPv6 was
using a mixture of xchg() and sk_dst_lock to protect concurrent changes
to sk->sk_dst_cache, leading to possible corruptions and crashes.

ip6_sk_dst_lookup_flow() uses sk_dst_check() anyway, so the simplest
way to fix the mess is to remove sk_dst_lock completely, as we did for
IPv4.

__ip6_dst_store() and ip6_dst_store() share same implementation.

sk_setup_caps() being called with socket lock being held or not,
we have to use sk_dst_set() instead of __sk_dst_set()

Note that I had to move the "np->dst_cookie = rt6_get_cookie(rt);"
in ip6_dst_store() before the sk_setup_caps(sk, dst) call.

This is because ip6_dst_store() can be called from process context,
without any lock held.

As soon as the dst is installed in sk->sk_dst_cache, dst can be freed
from another cpu doing a concurrent ip6_dst_store()

Doing the dst dereference before doing the install is needed to make
sure no use after free would trigger.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 13 deletions

diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 2bfb2ad2fab1..877f682989b8 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -133,27 +133,18 @@ void rt6_clean_tohost(struct net *net, struct in6_addr *gateway);
 /*
  *	Store a destination cache entry in a socket
  */
-static inline void __ip6_dst_store(struct sock *sk, struct dst_entry *dst,
-				   const struct in6_addr *daddr,
-				   const struct in6_addr *saddr)
+static inline void ip6_dst_store(struct sock *sk, struct dst_entry *dst,
+				 const struct in6_addr *daddr,
+				 const struct in6_addr *saddr)
 {
 	struct ipv6_pinfo *np = inet6_sk(sk);
-	struct rt6_info *rt = (struct rt6_info *) dst;
 
+	np->dst_cookie = rt6_get_cookie((struct rt6_info *)dst);
 	sk_setup_caps(sk, dst);
 	np->daddr_cache = daddr;
 #ifdef CONFIG_IPV6_SUBTREES
 	np->saddr_cache = saddr;
 #endif
-	np->dst_cookie = rt6_get_cookie(rt);
-}
-
-static inline void ip6_dst_store(struct sock *sk, struct dst_entry *dst,
-				 struct in6_addr *daddr, struct in6_addr *saddr)
-{
-	spin_lock(&sk->sk_dst_lock);
-	__ip6_dst_store(sk, dst, daddr, saddr);
-	spin_unlock(&sk->sk_dst_lock);
 }
 
 static inline bool ipv6_unicast_destination(const struct sk_buff *skb)