[PATCH nf] netfilter: nf_tables: inconditionally bump set->nelems before insertion

[Pablo Neira Ayuso <pablo@netfilter.org>]

In case that the set is full, a new element gets published then removed
without waiting for the RCU grace period, while RCU reader can be
walking over it already.

To address this issue, add the element transaction even if set is full,
but toggle the set_full flag to report -ENFILE so the abort path safely
unwinds the set to its previous state.

As for element updates, decrement set->nelems to restore it.

Fixes: 35d0ac9070ef ("netfilter: nf_tables: fix set->nelems counting with no NLM_F_EXCL")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 net/netfilter/nf_tables_api.c | 30 ++++++++++++++++--------------
 1 file changed, 16 insertions(+), 14 deletions(-)

diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
index 0c5a4855b97d..834736237b09 100644
--- a/net/netfilter/nf_tables_api.c
+++ b/net/netfilter/nf_tables_api.c
@@ -7171,6 +7171,7 @@ static int nft_add_set_elem(struct nft_ctx *ctx, struct nft_set *set,
 	struct nft_data_desc desc;
 	enum nft_registers dreg;
 	struct nft_trans *trans;
+	bool set_full = false;
 	u64 expiration;
 	u64 timeout;
 	int err, i;
@@ -7462,10 +7463,18 @@ static int nft_add_set_elem(struct nft_ctx *ctx, struct nft_set *set,
 	if (err < 0)
 		goto err_elem_free;
 
+	if (!(flags & NFT_SET_ELEM_CATCHALL)) {
+		unsigned int max = nft_set_maxsize(set), nelems;
+
+		nelems = atomic_inc_return(&set->nelems);
+		if (nelems > max)
+			set_full = true;
+	}
+
 	trans = nft_trans_elem_alloc(ctx, NFT_MSG_NEWSETELEM, set);
 	if (trans == NULL) {
 		err = -ENOMEM;
-		goto err_elem_free;
+		goto err_set_size;
 	}
 
 	ext->genmask = nft_genmask_cur(ctx->net);
@@ -7517,7 +7526,7 @@ static int nft_add_set_elem(struct nft_ctx *ctx, struct nft_set *set,
 
 						ue->priv = elem_priv;
 						nft_trans_commit_list_add_elem(ctx->net, trans);
-						goto err_elem_free;
+						goto err_set_size;
 					}
 				}
 			}
@@ -7535,23 +7544,16 @@ static int nft_add_set_elem(struct nft_ctx *ctx, struct nft_set *set,
 		goto err_element_clash;
 	}
 
-	if (!(flags & NFT_SET_ELEM_CATCHALL)) {
-		unsigned int max = nft_set_maxsize(set);
-
-		if (!atomic_add_unless(&set->nelems, 1, max)) {
-			err = -ENFILE;
-			goto err_set_full;
-		}
-	}
-
 	nft_trans_container_elem(trans)->elems[0].priv = elem.priv;
 	nft_trans_commit_list_add_elem(ctx->net, trans);
-	return 0;
 
-err_set_full:
-	nft_setelem_remove(ctx->net, set, elem.priv);
+	return set_full ? -ENFILE : 0;
+
 err_element_clash:
 	kfree(trans);
+err_set_size:
+	if (!(flags & NFT_SET_ELEM_CATCHALL) && !set_full)
+		atomic_dec(&set->nelems);
 err_elem_free:
 	nf_tables_set_elem_destroy(ctx, set, elem.priv);
 err_parse_data:
-- 
2.47.3