netfilter: nf_conntrack: fix a race in __nf_conntrack_confirm against nf_ct_get_next_corpse()

This race was triggered by a 'conntrack -F' command running in parallel
to the insertion of a hash for a new connection. Losing this race led to
a dead conntrack entry effectively blocking traffic for a particular
connection until timeout or flushing the conntrack hashes again.
Now the check for an already dying connection is done inside the lock.

Signed-off-by: Joerg Marx <joerg.marx@secunet.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

2 files changed, 11 insertions, 1 deletions

diff --git a/include/net/netfilter/nf_conntrack_core.h b/include/net/netfilter/nf_conntrack_core.h
index dffde8e6920e..3d7524fba194 100644
--- a/include/net/netfilter/nf_conntrack_core.h
+++ b/include/net/netfilter/nf_conntrack_core.h
@@ -61,7 +61,7 @@ static inline int nf_conntrack_confirm(struct sk_buff *skb)
         int ret = NF_ACCEPT;
 
         if (ct && ct != &nf_conntrack_untracked) {
-                if (!nf_ct_is_confirmed(ct) && !nf_ct_is_dying(ct))
+                if (!nf_ct_is_confirmed(ct))
                         ret = __nf_conntrack_confirm(skb);
                 if (likely(ret == NF_ACCEPT))
                         nf_ct_deliver_cached_events(ct);
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index b83c530c5e0a..eeeb8bc73982 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -424,6 +424,16 @@ __nf_conntrack_confirm(struct sk_buff *skb)
 
         spin_lock_bh(&nf_conntrack_lock);
 
+        /* We have to check the DYING flag inside the lock to prevent
+           a race against nf_ct_get_next_corpse() possibly called from
+           user context, else we insert an already 'dead' hash, blocking
+           further use of that particular connection -JM */
+
+        if (unlikely(nf_ct_is_dying(ct))) {
+                spin_unlock_bh(&nf_conntrack_lock);
+                return NF_ACCEPT;
+        }
+
         /* See if there's one in the list already, including reverse:
            NAT could have grabbed it without realizing, since we're
            not in the hash.  If there is, we lost race. */