rhashtable: Fix unprotected RCU dereference in __rht_ptr

The rcu_dereference call in rht_ptr_rcu is completely bogus because
we've already dereferenced the value in __rht_ptr and operated on it.
This causes potential double readings which could be fatal.  The RCU
dereference must occur prior to the comparison in __rht_ptr.

This patch changes the order of RCU dereference so that it is done
first and the result is then fed to __rht_ptr.  The RCU marking
changes have been minimised using casts which will be removed in
a follow-up patch.

Fixes: ba6306e3f6 ("rhashtable: Remove RCU marking from...")
Reported-by: "Gong, Sishuai" <sishuai@purdue.edu>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/rhashtable.h

@@ -349,11 +349,11 @@ static inline void rht_unlock(struct bucket_table *tbl,
 	local_bh_enable();
 }
 
-static inline struct rhash_head __rcu *__rht_ptr(
-	struct rhash_lock_head *const *bkt)
+static inline struct rhash_head *__rht_ptr(
+	struct rhash_lock_head *p, struct rhash_lock_head __rcu *const *bkt)
 {
-	return (struct rhash_head __rcu *)
-		((unsigned long)*bkt & ~BIT(0) ?:
+	return (struct rhash_head *)
+		((unsigned long)p & ~BIT(0) ?:
 		 (unsigned long)RHT_NULLS_MARKER(bkt));
 }
 
@@ -365,25 +365,26 @@ static inline struct rhash_head __rcu *__rht_ptr(
  *            access is guaranteed, such as when destroying the table.
  */
 static inline struct rhash_head *rht_ptr_rcu(
-	struct rhash_lock_head *const *bkt)
+	struct rhash_lock_head *const *p)
 {
-	struct rhash_head __rcu *p = __rht_ptr(bkt);
-
-	return rcu_dereference(p);
+	struct rhash_lock_head __rcu *const *bkt = (void *)p;
+	return __rht_ptr(rcu_dereference(*bkt), bkt);
 }
 
 static inline struct rhash_head *rht_ptr(
-	struct rhash_lock_head *const *bkt,
+	struct rhash_lock_head *const *p,
 	struct bucket_table *tbl,
 	unsigned int hash)
 {
-	return rht_dereference_bucket(__rht_ptr(bkt), tbl, hash);
+	struct rhash_lock_head __rcu *const *bkt = (void *)p;
+	return __rht_ptr(rht_dereference_bucket(*bkt, tbl, hash), bkt);
 }
 
 static inline struct rhash_head *rht_ptr_exclusive(
-	struct rhash_lock_head *const *bkt)
+	struct rhash_lock_head *const *p)
 {
-	return rcu_dereference_protected(__rht_ptr(bkt), 1);
+	struct rhash_lock_head __rcu *const *bkt = (void *)p;
+	return __rht_ptr(rcu_dereference_protected(*bkt, 1), bkt);
 }
 
 static inline void rht_assign_locked(struct rhash_lock_head **bkt,