[PATCH net 1/2] net/sched: act_api: reject duplicate actions in a batch

tca_action_gd() builds actions[] with one tcf_action_get_1() per nested
TCA_ACT_TAB entry.  Each successful lookup takes its own reference to the
resolved action, so repeated TCA_ACT_INDEX entries in one request yield
the same pointer in multiple slots.

For RTM_DELACTION, tcf_action_delete() then consumes two references per
slot: one in tcf_action_put() and one in tcf_idr_delete_index().  A
duplicate therefore drives the refcount to zero mid-walk -- the delete
frees the action and removes its IDR slot -- and the next slot calls
tcf_action_put() on the freed action:

  refcount_t: underflow; use-after-free.
  WARNING: lib/refcount.c:87 at refcount_dec_not_one
   refcount_dec_and_mutex_lock
   __tcf_action_put
   tca_action_gd

Three duplicate entries are enough.  Reject a repeated action while a
delete batch is built, dropping the reference the extra lookup took, and
return -EINVAL.  RTM_GETACTION balances its own references and keeps
accepting duplicate entries.

Conditions to recreate the bug:
  tc actions add action gact index 100
  tc actions delete action gact index 100 action gact index 100 \
                     action gact index 100

Fixes: 16af6067392c ("net: sched: implement reference counted action release")
Reported-by: Sashiko (gemini) <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260824153903.4143642-1-victor@mojatatu.com
Tested-by: hybris <redacted>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/sched/act_api.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/net/sched/act_api.c b/net/sched/act_api.c
index e45a63be397c..a9323c42a69a 100644
--- a/net/sched/act_api.c
+++ b/net/sched/act_api.c

@@ -2026,7 +2026,7 @@ static int
 tca_action_gd(struct net *net, struct nlattr *nla, struct nlmsghdr *n,
 	      u32 portid, int event, struct netlink_ext_ack *extack)
 {
-	int i, ret;
+	int i, j, ret;
 	struct nlattr *tb[TCA_ACT_MAX_PRIO + 1];
 	struct tc_action *act;
 	size_t attr_size = 0;

@@ -2051,6 +2051,25 @@ tca_action_gd(struct net *net, struct nlattr *nla, struct nlmsghdr *n,
 			ret = PTR_ERR(act);
 			goto err;
 		}
+
+		/* A delete consumes two references per slot (tcf_action_put()
+		 * and tcf_idr_delete_index()) but each entry takes one, so a
+		 * repeated action would hit zero mid-walk.  GET balances its
+		 * own references and keeps accepting duplicates.
+		 */
+		if (event == RTM_DELACTION) {
+			for (j = 0; j < i - 1; j++) {
+				if (actions[j] != act)
+					continue;
+
+				tcf_action_put(act);
+				NL_SET_ERR_MSG(extack,
+					       "Duplicate TC action in a delete batch");
+				ret = -EINVAL;
+				goto err;
+			}
+		}
+
 		attr_size += tcf_action_fill_size(act);
 		actions[i - 1] = act;
 	}

-- 
2.43.0