net: fix a race in dst_release()

[ Upstream commit d69bbf88c8d0b367cf3e3a052f6daadf630ee566 ]

Only cpu seeing dst refcount going to 0 can safely
dereference dst->flags.

Otherwise an other cpu might already have freed the dst.

Fixes: 27b75c95f10d ("net: avoid RCU for NOCACHE dst")
Reported-by: Greg Thelen <gthelen@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/core/dst.c b/net/core/dst.c
index 002144bea93517d7e2e5b2c0ac00e70c028174a2..cc4a086ae09c3419dedaae726ddb1a05ceb3615b 100644 (file)
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -287,7 +287,7 @@ void dst_release(struct dst_entry *dst)
                if (unlikely(newrefcnt < 0))
                        net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
                                             __func__, dst, newrefcnt);
-               if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt)
+               if (!newrefcnt && unlikely(dst->flags & DST_NOCACHE))
                        call_rcu(&dst->rcu_head, dst_destroy_rcu);
        }
 }