netfilter: nft_set_rbtree: fix null deref on element insertion

There is no guarantee that rb_prev() will not return NULL in nft_rbtree_gc_elem():

general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
 nft_add_set_elem+0x14b0/0x2990
  nf_tables_newsetelem+0x528/0xb30

Furthermore, there is a possible use-after-free while iterating,
'node' can be free'd so we need to cache the next value to use.

Fixes: c9e6978e ("netfilter: nft_set_rbtree: Switch to node list walk for overlap detection")
Signed-off-by: Florian Westphal <fw@strlen.de>

net/netfilter/nft_set_rbtree.c

@@ -221,7 +221,7 @@ static int nft_rbtree_gc_elem(const struct nft_set *__set,
 {
 	struct nft_set *set = (struct nft_set *)__set;
 	struct rb_node *prev = rb_prev(&rbe->node);
-	struct nft_rbtree_elem *rbe_prev;
+	struct nft_rbtree_elem *rbe_prev = NULL;
 	struct nft_set_gc_batch *gcb;
 
 	gcb = nft_set_gc_batch_check(set, NULL, GFP_ATOMIC);
@@ -229,17 +229,21 @@ static int nft_rbtree_gc_elem(const struct nft_set *__set,
 		return -ENOMEM;
 
 	/* search for expired end interval coming before this element. */
-	do {
+	while (prev) {
 		rbe_prev = rb_entry(prev, struct nft_rbtree_elem, node);
 		if (nft_rbtree_interval_end(rbe_prev))
 			break;
 
 		prev = rb_prev(prev);
-	} while (prev != NULL);
+	}
+
+	if (rbe_prev) {
+		rb_erase(&rbe_prev->node, &priv->root);
+		atomic_dec(&set->nelems);
+	}
 
-	rb_erase(&rbe_prev->node, &priv->root);
 	rb_erase(&rbe->node, &priv->root);
-	atomic_sub(2, &set->nelems);
+	atomic_dec(&set->nelems);
 
 	nft_set_gc_batch_add(gcb, rbe);
 	nft_set_gc_batch_complete(gcb);
@@ -268,7 +272,7 @@ static int __nft_rbtree_insert(const struct net *net, const struct nft_set *set,
 			       struct nft_set_ext **ext)
 {
 	struct nft_rbtree_elem *rbe, *rbe_le = NULL, *rbe_ge = NULL;
-	struct rb_node *node, *parent, **p, *first = NULL;
+	struct rb_node *node, *next, *parent, **p, *first = NULL;
 	struct nft_rbtree *priv = nft_set_priv(set);
 	u8 genmask = nft_genmask_next(net);
 	int d, err;
@@ -307,7 +311,9 @@ static int __nft_rbtree_insert(const struct net *net, const struct nft_set *set,
 	 * Values stored in the tree are in reversed order, starting from
 	 * highest to lowest value.
 	 */
-	for (node = first; node != NULL; node = rb_next(node)) {
+	for (node = first; node != NULL; node = next) {
+		next = rb_next(node);
+
 		rbe = rb_entry(node, struct nft_rbtree_elem, node);
 
 		if (!nft_set_elem_active(&rbe->ext, genmask))