[PATCH v2] ipv4: synchronize bind() with RTM_NEWADDR notifications

["Timo Teräs" <timo.teras@iki.fi>]

Otherwise we have race condition to user land:
 1. process A: changes IP address
 2. process A: kernel sends RTM_NEWADDR (and schedules out)
 3. process B: gets notification
 4. process B: tries to bind() to new IP, but fails with EADDRNOTAVAIL
      because FIB is not yet updated and inet_addr_type() in inet_bind()
      does not recognize the IP as local
 5. process A: calls inetaddr_chain notifiers which updates FIB

Fix the error path to synchronize with configuration changes and retry
the address type check.

IPv6 side seems to handle the notifications properly: bind() immediately
after RTM_NEWADDR succeeds as expected.  This is because ipv6_chk_addr()
uses inet6_addr_lst which is updated before address notification.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
---
Since there was no reply to my question if this is ok, I interpreted it
as "maybe". So here's the code for review. Hopefully this helps determining
if this is an acceptable fix.

 net/ipv4/af_inet.c  |   18 ++++++++++++++++--
 net/ipv6/af_inet6.c |   13 ++++++++++---
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 6a1100c..013ab11 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -481,8 +481,22 @@ int inet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 	    addr->sin_addr.s_addr != htonl(INADDR_ANY) &&
 	    chk_addr_ret != RTN_LOCAL &&
 	    chk_addr_ret != RTN_MULTICAST &&
-	    chk_addr_ret != RTN_BROADCAST)
-		goto out;
+	    chk_addr_ret != RTN_BROADCAST) {
+		/* inet_addr_type() does a FIB lookup to check the
+		 * address type. Since FIB is updated after sending
+		 * RTM_NEWADDR notification, an application may end up
+		 * doing bind() before the FIB is updated. To avoid
+		 * returning a false negative, wait for possible ongoing
+		 * address changes to finish by acquiring rtnl lock and
+		 * retry the address type lookup. */
+		rtnl_lock();
+		rtnl_unlock();
+		chk_addr_ret = inet_addr_type(sock_net(sk), addr->sin_addr.s_addr);
+		if (chk_addr_ret != RTN_LOCAL &&
+		    chk_addr_ret != RTN_MULTICAST &&
+		    chk_addr_ret != RTN_BROADCAST)
+			goto out;
+	}
 
 	snum = ntohs(addr->sin_port);
 	err = -EACCES;
diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
index 56b9bf2..b1a83e1 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -300,7 +300,7 @@ int inet6_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 			goto out;
 		}
 
-		/* Reproduce AF_INET checks to make the bindings consitant */
+		/* Reproduce AF_INET checks to make the bindings consistent */
 		v4addr = addr->sin6_addr.s6_addr32[3];
 		chk_addr_ret = inet_addr_type(net, v4addr);
 		if (!sysctl_ip_nonlocal_bind &&
@@ -309,8 +309,15 @@ int inet6_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 		    chk_addr_ret != RTN_LOCAL &&
 		    chk_addr_ret != RTN_MULTICAST &&
 		    chk_addr_ret != RTN_BROADCAST) {
-			err = -EADDRNOTAVAIL;
-			goto out;
+			rtnl_lock();
+			rtnl_unlock();
+			chk_addr_ret = inet_addr_type(net, v4addr);
+			if (chk_addr_ret != RTN_LOCAL &&
+			    chk_addr_ret != RTN_MULTICAST &&
+			    chk_addr_ret != RTN_BROADCAST) {
+				err = -EADDRNOTAVAIL;
+				goto out;
+			}
 		}
 	} else {
 		if (addr_type != IPV6_ADDR_ANY) {
-- 
1.7.1