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net/sched: cls_u32: fix duplicate handle when node ID pool is exhausted

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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: 7801db8aec ("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>
Link: https://patch.msgid.link/20260825081052.133898-1-jhs@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jamal Hadi Salim 2026-08-25 04:10:51 -04:00 committed by Jakub Kicinski
parent f05f0d85cd
commit d7e7e98d23
1 changed files with 26 additions and 6 deletions
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32
net/sched/cls_u32.c
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@ -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) {