CVE-2026-98629: net/sched: cls_u32: fix manual hash table handle IDR aliasing
In the Linux kernel, the following vulnerability has been resolved:
net/sched: clsu32: fix manual hash table handle IDR aliasing
A u32 hash table created with an explicit handle ('tc filter add ... handle 801: u32 divisor N') keys its IDR entry on the raw handle, while the destroy paths free it under handle2id(handle). The two key domains disagree for handles in the 0x800..0xFFF htid range: handle2id() folds them back into the auto-allocated id space (1..0x7FF).
A manual table therefore leaves its raw-keyed IDR entry unreachable on delete (a permanent leak), and its delete can drop the idr entry of an unrelated live auto table. A later auto allocation can then hand out a handle that aliases the live manual table; u32lookupht() first-match routes lookups and TCAU32LINK for that htid to the wrong table.
Key the divisor-path alloc on handle2id(handle) so allocation and removal share one key domain. A manual handle that maps onto an id already in use is rejected with -ENOSPC, and auto allocation skips ids held by live manual tables.
Conditions to recreate: ip link add test0 type dummy tc qdisc add dev test0 clsact tc filter add dev test0 ingress protocol ip pref 1 \ handle 801: u32 divisor 16 tc filter add dev test0 ingress protocol ip pref 2 u32 divisor 16 tc -d filter show dev test0 ingress | grep 'fh 801:' # unpatched: two live tables with handle 0x80100000 (the pref 2 root # hnode is auto-allocated id 1); patched: the auto hnode takes id 2.
Also tested with a poc with a live u32 table on the block, add/delete a manual table 'handle 901: u32 divisor 1' twice; unpatched, the re-add fails with -ENOSPC because the raw key leaked on the first delete.
Affected Software
Event History
Frequently Asked Questions
What configurations are exposed to the aliasing problem?
The issue requires cls_u32 hash tables created with explicit manual handles in the 0x800 through 0xFFF htid range. It involves interaction with automatically allocated u32 hash-table IDs; the provided recreation uses an ingress clsact qdisc on a dummy interface.
What can happen if the vulnerable behavior is triggered?
Deleting a manually handled table can leave its own raw-keyed IDR entry permanently leaked and can remove the IDR entry for an unrelated live automatically allocated table. A later automatic allocation may receive an aliasing handle, causing u32 lookup and TCA_U32_LINK operations for that htid to be routed to the wrong table.
How does the fix change behavior for manual handles?
The fix allocates divisor-path entries using handle2id(handle), matching the key domain used for removal. A manual handle that maps to an ID already in use is rejected with -ENOSPC, and automatic allocation skips IDs held by live manual tables.