[PATCH net v2 1/2 RESEND] net/sched: cls_u32: fix duplicate handle when node ID pool is exhausted

[PATCH net v2 1/2 RESEND] net/sched: cls_u32: fix duplicate handle when node ID pool is exhausted

[Jamal Hadi Salim]

gen_new_kid() falls back to returning max (htid | 0xFFF) when both
idr_alloc_u32() ranges are full, instead of reporting an error.
u32_change() trusts that value and inserts a new knode with a handle
that is already live in the hash table, breaking handle uniqueness
within the table's node ID space.

The handle was never reserved in ht->handle_idr, so every later error
path that does idr_remove(&ht->handle_idr, handle) removes the
reservation of a different, live knode, which is then reused — one
failed add compounds into further duplicates.

The 4095 limit is per (table, bucket) — ht->handle_idr is per hash
table and the range is derived from htid (bucketid), so a table with
divisor 256 can legitimately hold 256*4095 knodes.

The sibling helper gen_new_htid() has the same silent in-band failure:
it returns 0 when the tp_c handle pool (1..0x7FF) is full, and
u32_init() publishes the root hash table with handle 0 without
checking.  Two root tables with handle 0 alias in u32_lookup_ht(),
allowing cross-tcf_proto knode add/lookup/delete.  Add the same
exhaustion check that the divisor path already has.

Return an error so u32_change() fails with ENOSPC/ENOMEM when the
node ID space is exhausted, and so u32_init() fails with -ENOMEM
when the hash table ID space is exhausted.

Conditions to recreate the bug:
- CONFIG_NET_SCHED=y, CONFIG_CLS_U32=y (or =m with module loaded)
- Create a clsact qdisc on a device, then add 4095 u32 filters with
  auto-generated handles to fill the node ID space for the root hash
  table (single bucket). The 4096th auto-handle filter add triggers
  the duplicate handle (fh 800::fff reused). Reachable at Level 2
  (unshare -Urn, namespace-local CAP_NET_ADMIN).
- For gen_new_htid: create 2047 u32 proto entries on the same block
  to fill the tp_c handle pool, then create one more. The root table
  gets handle 0 and aliases with other handle-0 root tables.

Fixes: 7801db8aec95 ("net_sched: avoid generating same handle for u32 filters")
Reported-by: vega@nebusec.ai
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
v1 -> v2:
- Commit message: removed incorrect "4095-knode cap" claim (knodes is a
  liveness counter, not a limit; 4095 is per (table, bucket) via
  ht->handle_idr/htid); reworded to "handle uniqueness within node ID
  space".
- Commit message: added IDR-desync paragraph (max handle never reserved
  -> erridr removes other live knode's reservation -> reuse compounds).
- Commit message: noted ENOSPC/ENOMEM both reachable (idr_get_free/
  radix_tree_extend).
- Fixes: e7614370d6f04 -> 7801db8aec95 (duplicate-handle bug predates
  the IDR conversion; e761 only added the IDR-desync consequence).
- Added NL_SET_ERR_MSG_MOD(extack, "Hash table node ID pool exhausted")
  at both gen_new_kid() call sites; bare -ENOSPC -> "No space left on
  device" was confusing.
- Folded gen_new_htid() exhaustion check in u32_init() (Sashiko nipa
  main-1-0): same silent in-band failure pattern — gen_new_htid()
  returns 0 when tp_c handle pool full, u32_init() published root table
  with handle 0 without checking, causing cross-tcf_proto aliasing.
- Renewed tdc test (v1 tdc failed in long-running sweep, prompting v2).
---
 net/sched/cls_u32.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index c297d7dbcf91..1a72817bd449 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -370,6 +370,10 @@ static int u32_init(struct tcf_proto *tp)
 
 	refcount_set(&root_ht->refcnt, 1);
 	root_ht->handle = tp_c ? gen_new_htid(tp_c, root_ht) : id2handle(0);
+	if (root_ht->handle == 0) {
+		kfree(root_ht);
+		return -ENOMEM;
+	}
 	root_ht->prio = tp->prio;
 	root_ht->is_root = true;
 	idr_init(&root_ht->handle_idr);
@@ -695,16 +699,19 @@ static int u32_delete(struct tcf_proto *tp, void *arg, bool *last,
 	return ret;
 }
 
-static u32 gen_new_kid(struct tc_u_hnode *ht, u32 htid)
+static u32 gen_new_kid(struct tc_u_hnode *ht, u32 htid, int *err)
 {
 	u32 index = htid | 0x800;
 	u32 max = htid | 0xFFF;
 
+	*err = 0;
+
 	if (idr_alloc_u32(&ht->handle_idr, NULL, &index, max, GFP_KERNEL)) {
 		index = htid + 1;
-		if (idr_alloc_u32(&ht->handle_idr, NULL, &index, max,
-				 GFP_KERNEL))
-			index = max;
+		*err = idr_alloc_u32(&ht->handle_idr, NULL, &index, max,
+				     GFP_KERNEL);
+		if (*err)
+			return 0;
 	}
 
 	return index;
@@ -1079,7 +1086,11 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 		 * handle which is used to uniquely identify the match entry.
 		 */
 		if (!TC_U32_NODE(handle)) {
-			handle = gen_new_kid(ht, htid);
+			handle = gen_new_kid(ht, htid, &err);
+			if (err) {
+				NL_SET_ERR_MSG_MOD(extack, "Hash table node ID pool exhausted");
+				return err;
+			}
 		} else {
 			handle = htid | TC_U32_NODE(handle);
 			err = idr_alloc_u32(&ht->handle_idr, NULL, &handle,
@@ -1091,7 +1102,11 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 		/* The user did not give us a handle; lets just generate one
 		 * from the table's pool of nodeids.
 		 */
-		handle = gen_new_kid(ht, htid);
+		handle = gen_new_kid(ht, htid, &err);
+		if (err) {
+			NL_SET_ERR_MSG_MOD(extack, "Hash table node ID pool exhausted");
+			return err;
+		}
 	}
 
 	if (tb[TCA_U32_SEL] == NULL) {
-- 
2.43.0

[PATCH net v2 2/2 RESEND] selftests: tc-testing: add u32 node ID pool exhaustion test

[Jamal Hadi Salim]

Add a tdc test case that fills the u32 node ID space with 4095
auto-generated handles, then attempts to add a 4096th. On the fixed
kernel the 4096th filter is rejected with ENOSPC (exit 2). On the
unfixed kernel it silently succeeds with a duplicate handle.

Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 .../tc-testing/tc-tests/filters/u32.json      | 24 +++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json b/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json
index b2ca9d4e991b..039bb8278a60 100644
--- a/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json
+++ b/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json
@@ -353,5 +353,29 @@
         "teardown": [
             "$TC qdisc del dev $DEV1 parent root drr"
         ]
+    },
+    {
+        "id": "934a",
+        "name": "Add u32 filter when node ID pool is exhausted (4096th filter rejected)",
+        "category": [
+            "filter",
+            "u32"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "$TC qdisc add dev $DUMMY clsact",
+            "bash -c 'yes \"filter add dev $DUMMY ingress prio 1 protocol ip u32 match u8 0 0 at 0\" | head -n 4095 > /tmp/u32_batch.txt'",
+            "bash -c '$TC -b /tmp/u32_batch.txt'"
+        ],
+        "cmdUnderTest": "$TC filter add dev $DUMMY ingress prio 1 protocol ip u32 match u8 0 0 at 0",
+        "expExitCode": "2",
+        "verifyCmd": "$TC -d filter show dev $DUMMY ingress",
+        "matchPattern": "fh 800::",
+        "matchCount": "4095",
+        "teardown": [
+            "$TC qdisc del dev $DUMMY clsact"
+        ]
     }
 ]
-- 
2.43.0

Re: [PATCH net v2 2/2 RESEND] selftests: tc-testing: add u32 node ID pool exhaustion test

[Jamal Hadi Salim]

On Sat, Aug 22, 2026 at 6:21 PM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
>
> Add a tdc test case that fills the u32 node ID space with 4095
> auto-generated handles, then attempts to add a 4096th. On the fixed
> kernel the 4096th filter is rejected with ENOSPC (exit 2). On the
> unfixed kernel it silently succeeds with a duplicate handle.
>
> Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>

pw-bot: cr

cheers,
jamal

> ---
>  .../tc-testing/tc-tests/filters/u32.json      | 24 +++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json b/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json
> index b2ca9d4e991b..039bb8278a60 100644
> --- a/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json
> +++ b/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json
> @@ -353,5 +353,29 @@
>          "teardown": [
>              "$TC qdisc del dev $DEV1 parent root drr"
>          ]
> +    },
> +    {
> +        "id": "934a",
> +        "name": "Add u32 filter when node ID pool is exhausted (4096th filter rejected)",
> +        "category": [
> +            "filter",
> +            "u32"
> +        ],
> +        "plugins": {
> +            "requires": "nsPlugin"
> +        },
> +        "setup": [
> +            "$TC qdisc add dev $DUMMY clsact",
> +            "bash -c 'yes \"filter add dev $DUMMY ingress prio 1 protocol ip u32 match u8 0 0 at 0\" | head -n 4095 > /tmp/u32_batch.txt'",
> +            "bash -c '$TC -b /tmp/u32_batch.txt'"
> +        ],
> +        "cmdUnderTest": "$TC filter add dev $DUMMY ingress prio 1 protocol ip u32 match u8 0 0 at 0",
> +        "expExitCode": "2",
> +        "verifyCmd": "$TC -d filter show dev $DUMMY ingress",
> +        "matchPattern": "fh 800::",
> +        "matchCount": "4095",
> +        "teardown": [
> +            "$TC qdisc del dev $DUMMY clsact"
> +        ]
>      }
>  ]
> --
> 2.43.0
>