ipv6: fix race condition regarding dst->expires and dst->from.

Eric Dumazet wrote:
| Some strange crashes happen in rt6_check_expired(), with access
| to random addresses.
|
| At first glance, it looks like the RTF_EXPIRES and
| stuff added in commit 1716a96101c49186b
| (ipv6: fix problem with expired dst cache)
| are racy : same dst could be manipulated at the same time
| on different cpus.
|
| At some point, our stack believes rt->dst.from contains a dst pointer,
| while its really a jiffie value (as rt->dst.expires shares the same area
| of memory)
|
| rt6_update_expires() should be fixed, or am I missing something ?
|
| CC Neil because of https://bugzilla.redhat.com/show_bug.cgi?id=892060

Because we do not have any locks for dst_entry, we cannot change
essential structure in the entry; e.g., we cannot change reference
to other entity.

To fix this issue, split 'from' and 'expires' field in dst_entry
out of union.  Once it is 'from' is assigned in the constructor,
keep the reference until the very last stage of the life time of
the object.

Of course, it is unsafe to change 'from', so make rt6_set_from simple
just for fresh entries.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: Neil Horman <nhorman@tuxdriver.com>
CC: Gao Feng <gaofeng@cn.fujitsu.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reported-by: Steinar H. Gunderson <sesse@google.com>
Reviewed-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 14 insertions, 33 deletions

diff --git a/include/net/dst.h b/include/net/dst.h
index 3da47e0a4a1..853cda11e51 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -36,13 +36,9 @@ struct dst_entry {
 	struct net_device       *dev;
 	struct  dst_ops	        *ops;
 	unsigned long		_metrics;
-	union {
-		unsigned long           expires;
-		/* point to where the dst_entry copied from */
-		struct dst_entry        *from;
-	};
+	unsigned long           expires;
 	struct dst_entry	*path;
-	void			*__pad0;
+	struct dst_entry	*from;
 #ifdef CONFIG_XFRM
 	struct xfrm_state	*xfrm;
 #else
diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
index 6919a501f99..2a601e7da1b 100644
--- a/include/net/ip6_fib.h
+++ b/include/net/ip6_fib.h
@@ -164,50 +164,35 @@ static inline struct inet6_dev *ip6_dst_idev(struct dst_entry *dst)
 
 static inline void rt6_clean_expires(struct rt6_info *rt)
 {
-	if (!(rt->rt6i_flags & RTF_EXPIRES) && rt->dst.from)
-		dst_release(rt->dst.from);
-
 	rt->rt6i_flags &= ~RTF_EXPIRES;
-	rt->dst.from = NULL;
 }
 
 static inline void rt6_set_expires(struct rt6_info *rt, unsigned long expires)
 {
-	if (!(rt->rt6i_flags & RTF_EXPIRES) && rt->dst.from)
-		dst_release(rt->dst.from);
-
-	rt->rt6i_flags |= RTF_EXPIRES;
 	rt->dst.expires = expires;
+	rt->rt6i_flags |= RTF_EXPIRES;
 }
 
-static inline void rt6_update_expires(struct rt6_info *rt, int timeout)
+static inline void rt6_update_expires(struct rt6_info *rt0, int timeout)
 {
-	if (!(rt->rt6i_flags & RTF_EXPIRES)) {
-		if (rt->dst.from)
-			dst_release(rt->dst.from);
-		/* dst_set_expires relies on expires == 0 
-		 * if it has not been set previously.
-		 */
-		rt->dst.expires = 0;
-	}
-
-	dst_set_expires(&rt->dst, timeout);
-	rt->rt6i_flags |= RTF_EXPIRES;
+	struct rt6_info *rt;
+
+	for (rt = rt0; rt && !(rt->rt6i_flags & RTF_EXPIRES);
+	     rt = (struct rt6_info *)rt->dst.from);
+	if (rt && rt != rt0)
+		rt0->dst.expires = rt->dst.expires;
+
+	dst_set_expires(&rt0->dst, timeout);
+	rt0->rt6i_flags |= RTF_EXPIRES;
 }
 
 static inline void rt6_set_from(struct rt6_info *rt, struct rt6_info *from)
 {
 	struct dst_entry *new = (struct dst_entry *) from;
 
-	if (!(rt->rt6i_flags & RTF_EXPIRES) && rt->dst.from) {
-		if (new == rt->dst.from)
-			return;
-		dst_release(rt->dst.from);
-	}
-
 	rt->rt6i_flags &= ~RTF_EXPIRES;
-	rt->dst.from = new;
 	dst_hold(new);
+	rt->dst.from = new;
 }
 
 static inline void ip6_rt_put(struct rt6_info *rt)