[PATCH net v3 1/2] net/sched: cls_u32: fix duplicate handle when node ID pool is exhausted

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.  The extack message
distinguishes pool exhaustion (-ENOSPC) from a transient allocation
failure (-ENOMEM).

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: [email protected]
Tested-by: Victor Nogueira <[email protected]>
Signed-off-by: Jamal Hadi Salim <[email protected]>
---
v2 -> v3:
- Fixed tdc test that sashiko (correctly) pointed potential security
  issue on.
- extack: condition the "Hash table node ID pool exhausted" message on
  -ENOSPC; emit a neutral "Failed to allocate node ID" for -ENOMEM
  Introduce small extack helper. The v2 message was misleading for
  -ENOMEM (Sashiko nipa gpt-5-6-sol-1-2).

Sashiko links (v2 RESEND reviews that prompted v3):
- gemini: https://sashiko.dev/#/patchset/[email protected]
- nipa: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/[email protected]

v1 -> v2:
- Commit message: removed misleading "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"(sashiko).
- 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).

Sashiko links (v1 reviews that prompted v2):
- gemini: https://sashiko.dev/#/patchset/[email protected]
- nipa: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/[email protected]
---
 net/sched/cls_u32.c | 32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index ac6d0fa5a40e..a3e65c8cf29e 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);
@@ -695,21 +699,33 @@ 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;
 }
 
+static int u32_kid_extack(int err, struct netlink_ext_ack *extack)
+{
+	if (err == -ENOSPC)
+		NL_SET_ERR_MSG_MOD(extack, "Hash table node ID pool exhausted");
+	else
+		NL_SET_ERR_MSG_MOD(extack, "Failed to allocate node ID");
+	return err;
+}
+
 static const struct nla_policy u32_policy[TCA_U32_MAX + 1] = {
 	[TCA_U32_CLASSID]	= { .type = NLA_U32 },
 	[TCA_U32_HASH]		= { .type = NLA_U32 },
@@ -1079,7 +1095,9 @@ 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)
+				return u32_kid_extack(err, extack);
 		} else {
 			handle = htid | TC_U32_NODE(handle);
 			err = idr_alloc_u32(&ht->handle_idr, NULL, &handle,
@@ -1091,7 +1109,9 @@ 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)
+			return u32_kid_extack(err, extack);
 	}
 
 	if (tb[TCA_U32_SEL] == NULL) {
-- 
2.43.0