[PATCH net] net/sched: act_gate: reject oversized dumps instead of wrapping them

This is a follow-up to commit cfa165cbfbed ("net/sched: act_gate: budget
the per-entry list in get_fill_size") caught by Sashiko.
cfa165cbfbed sized the add/get reply from the action's real dump but did
not bound the entry count. An oversized gate therefore installs and its
dump emits a structurally corrupt message instead of failing cleanly.

Sashiko also flagged the enlarged reply as potentially something that
will crash the kernel with a memory-amplification / OOM vector.
We were able to recreate this using panic_on_oom=1 (128 concurent
threads GET on a VM sized at 1024M). In the past we have used
panic_on_oom=1; however,  I am weighing-in that: if i have to
create a crash using panic_on_oom=1 then that is a "hardening" issue
and therefore left to net-next. See the discussion with Jakub
(https://lore.kernel.org/netdev/20260914191108.55a1a4f1@kernel.org/ (local)).
Note: The issue is resolvable using an entry-count cap, but:
an entry-count cap would also reject gate configurations that work today,
so it cannot justify a stable backport - so policy cap is for net-next;
this patch closes only the malformed uAPI the sizing fix introduced.

parse_gate_list() accepts any number of TCA_GATE_ONE_ENTRY elements; the
only bound is the nlattr header, whose u16 nla_len caps the request at
~5460 minimal 12-byte entries. That is fine for the request, but
tcf_gate_dump() emits ~36 bytes per entry, so past ~1820 entries the
TCA_GATE_ENTRY_LIST nest exceeds U16_MAX and nla_nest_end() writes a
wrapped length. The enclosing TCA_ACT_OPTIONS and per-action nests wrap
the same way, and the outermost TCA_ACT_TAB wraps first, at ~1815
entries, so an oversized reply is already corrupt from there on.

Close those four wrap-capable nests with nla_nest_end_safe(). It returns
-EMSGSIZE instead of writing a wrapped length, and each site already has
an error label that trims and fails the dump. The output is byte
identical for every message that serializes; a message that would wrap
now fails through the same path that returned a clean error before
cfa165cbfbed, so no configuration that works today changes behavior.

Conditions to recreate the bug: CONFIG_NET_SCH_ACT_GATE=y; install a gate
with 1815 or more minimal TCA_GATE_ONE_ENTRY elements (12 bytes each on
the wire, only TCA_GATE_ENTRY_INTERVAL set) and dump it with
RTM_GETACTION. Up to 1814 entries the reply is well formed; from 1815 the
outermost TCA_ACT_TAB nest length wraps (1821 for the innermost
TCA_GATE_ENTRY_LIST). CONFIG_DEBUG_NET=y (default n) additionally trips a
WARN in nla_nest_end(). Installing the gate needs CAP_NET_ADMIN in a user
namespace; the RTM_GETACTION trigger needs no capability once the gate
exists.

Fixes: cfa165cbfbed ("net/sched: act_gate: budget the per-entry list in get_fill_size")
Reported-by: Sashiko (nipa) <sashiko-bot@kernel.org>
Closes: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260824153903.4143642-1-victor@mojatatu.com
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/sched/act_api.c  | 9 ++++++---
 net/sched/act_gate.c | 3 ++-
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/net/sched/act_api.c b/net/sched/act_api.c
index e45a63be397c..c7e87491a9fc 100644
--- a/net/sched/act_api.c
+++ b/net/sched/act_api.c

@@ -549,7 +549,8 @@ tcf_action_dump_1(struct sk_buff *skb, struct tc_action *a, int bind, int ref)
 		goto nla_put_failure;
 	err = tcf_action_dump_old(skb, a, bind, ref);
 	if (err > 0) {
-		nla_nest_end(skb, nest);
+		if (nla_nest_end_safe(skb, nest) < 0)
+			goto nla_put_failure;
 		return err;
 	}
 

@@ -1270,7 +1271,8 @@ int tcf_action_dump(struct sk_buff *skb, struct tc_action *actions[],
 			tcf_action_dump_1(skb, a, bind, ref);
 		if (err < 0)
 			goto errout;
-		nla_nest_end(skb, nest);
+		if (nla_nest_end_safe(skb, nest) < 0)
+			goto nla_put_failure;
 	}
 
 	return 0;

@@ -1684,7 +1686,8 @@ static int tca_get_fill(struct sk_buff *skb, struct tc_action *actions[],
 	if (tcf_action_dump(skb, actions, bind, ref, false) < 0)
 		goto out_nlmsg_trim;
 
-	nla_nest_end(skb, nest);
+	if (nla_nest_end_safe(skb, nest) < 0)
+		goto out_nlmsg_trim;
 
 	nlh->nlmsg_len = skb_tail_pointer(skb) - b;
 

diff --git a/net/sched/act_gate.c b/net/sched/act_gate.c
index 6d6d45e03c07..14801c604bd9 100644
--- a/net/sched/act_gate.c
+++ b/net/sched/act_gate.c

@@ -654,7 +654,8 @@ static int tcf_gate_dump(struct sk_buff *skb, struct tc_action *a,
 			goto nla_put_failure;
 	}
 
-	nla_nest_end(skb, entry_list);
+	if (nla_nest_end_safe(skb, entry_list) < 0)
+		goto nla_put_failure;
 
 	tcf_tm_dump(&t, &gact->tcf_tm);
 	if (nla_put_64bit(skb, TCA_GATE_TM, sizeof(t), &t, TCA_GATE_PAD))

-- 
2.43.0