[PATCH net] net/sched: cls_api: fix tp_created race losing existing tcf_proto

[PATCH net] net/sched: cls_api: fix tp_created race losing existing tcf_proto

[Aohan Mei]

tc_new_tfilter() attaches a filter to a chain. When no
tcf_proto (tp) exists for the given (protocol, prio), it creates one
with tcf_proto_create(), marks tp_created so the error path can clean
it up, and inserts it with tcf_chain_tp_insert_unique().

However, tcf_proto_create() can sleep, opening a race window in which a
concurrent thread may insert a tp with the same (protocol, prio). In
that case tcf_chain_tp_insert_unique() destroys tp_new and returns the
*existing* tp, but tp_created is never reset. If the request then fails
(e.g. kind mismatch), the errout path calls
tcf_chain_tp_delete_empty() on a tp this thread never created. For
classifiers without a delete_empty callback (all but cls_flower),
tcf_proto_check_delete() removes the tp unconditionally: a live tp and
all its filters are silently lost while the owner's change() still
reports success. The race is reachable because cls_flower on
ingress/clsact qdiscs runs without rtnl_lock and can interleave with
rtnl_lock-holding classifiers such as u32.

Fix this by making tcf_chain_tp_insert_unique() report through a new
"inserted" out-parameter whether tp_new was actually inserted, and
gate the errout delete_empty call on it instead of tp_created.
tp_created itself must stay set on this path: the chain reference was
consumed by the destroyed tp_new, and errout_tp relies on tp_created
to decide whether to tcf_chain_put(), so resetting it would underflow
the chain refcount.

Fixes: 8b64678e0af8 ("net: sched: refactor tp insert/delete for concurrent execution")
Cc: stable@vger.kernel.org
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Signed-off-by: Aohan Mei <henrymei@tencent.com>
---
 net/sched/cls_api.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index fee4524ad..9ceb2b538 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -1937,7 +1937,8 @@ static struct tcf_proto *tcf_chain_tp_find(struct tcf_chain *chain,
 static struct tcf_proto *tcf_chain_tp_insert_unique(struct tcf_chain *chain,
 						    struct tcf_proto *tp_new,
 						    u32 protocol, u32 prio,
-						    bool rtnl_held)
+						    bool rtnl_held,
+						    bool *inserted)
 {
 	struct tcf_chain_info chain_info;
 	struct tcf_proto *tp;
@@ -1948,6 +1949,7 @@ static struct tcf_proto *tcf_chain_tp_insert_unique(struct tcf_chain *chain,
 	if (tcf_proto_exists_destroying(chain, tp_new)) {
 		mutex_unlock(&chain->filter_chain_lock);
 		tcf_proto_destroy(tp_new, rtnl_held, false, NULL);
+		*inserted = false;
 		return ERR_PTR(-EAGAIN);
 	}
 
@@ -1964,6 +1966,11 @@ static struct tcf_proto *tcf_chain_tp_insert_unique(struct tcf_chain *chain,
 		tp_new = ERR_PTR(err);
 	}
 
+	/* Tell the caller whether tp_new was actually inserted, or an
+	 * already existing tp is being returned instead.
+	 */
+	*inserted = !tp && !err;
+
 	return tp_new;
 }
 
@@ -2254,11 +2261,13 @@ static int tc_new_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 	void *fh;
 	int err;
 	int tp_created;
+	bool tp_inserted;
 	bool rtnl_held = false;
 	u32 flags;
 
 replay:
 	tp_created = 0;
+	tp_inserted = false;
 
 	err = nlmsg_parse_deprecated(n, sizeof(*t), tca, TCA_MAX,
 				     rtm_tca_policy, extack);
@@ -2382,7 +2391,7 @@ static int tc_new_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 
 		tp_created = 1;
 		tp = tcf_chain_tp_insert_unique(chain, tp_new, protocol, prio,
-						rtnl_held);
+						rtnl_held, &tp_inserted);
 		if (IS_ERR(tp)) {
 			err = PTR_ERR(tp);
 			goto errout_tp;
@@ -2440,7 +2449,13 @@ static int tc_new_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 	}
 
 errout:
-	if (err && tp_created)
+	/* Only delete a tp that we actually inserted ourselves. When
+	 * tcf_chain_tp_insert_unique() raced with a concurrent insertion
+	 * it returns the existing tp; tp_created must stay set then (the
+	 * chain reference was consumed by the destroyed tp_new), but the
+	 * existing tp must not be deleted.
+	 */
+	if (err && tp_inserted)
 		tcf_chain_tp_delete_empty(chain, tp, rtnl_held, NULL);
 errout_tp:
 	if (chain) {
-- 
2.50.1 (Apple Git-155)