rhashtable: Fix use-after-free in rhashtable_walk_stop

The commit c4db8848af6af92f90462258603be844baeab44d ("rhashtable:
Move future_tbl into struct bucket_table") introduced a use-after-
free bug in rhashtable_walk_stop because it dereferences tbl after
droping the RCU read lock.

This patch fixes it by moving the RCU read unlock down to the bottom
of rhashtable_walk_stop.  In fact this was how I had it originally
but it got dropped while rearranging patches because this one
depended on the async freeing of bucket_table.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 3 deletions

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 9d53a46dcca9..b916679b3e3b 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -854,10 +854,8 @@ void rhashtable_walk_stop(struct rhashtable_iter *iter)
         struct rhashtable *ht;
         struct bucket_table *tbl = iter->walker->tbl;
 
-        rcu_read_unlock();
-
         if (!tbl)
-                return;
+                goto out;
 
         ht = iter->ht;
 
@@ -869,6 +867,9 @@ void rhashtable_walk_stop(struct rhashtable_iter *iter)
         mutex_unlock(&ht->mutex);
 
         iter->p = NULL;
+
+out:
+        rcu_read_unlock();
 }
 EXPORT_SYMBOL_GPL(rhashtable_walk_stop);