CVE-2025-38468

In the Linux kernel, the following vulnerability has been resolved:

net/sched: Return NULL when htblookupleaf encounters an empty rbtree

htblookupleaf has a BUG_ON that can trigger with the following:

tc qdisc del dev lo root tc qdisc add dev lo root handle 1: htb default 1 tc class add dev lo parent 1: classid 1:1 htb rate 64bit tc qdisc add dev lo parent 1:1 handle 2: netem tc qdisc add dev lo parent 2:1 handle 3: blackhole ping -I lo -c1 -W0.001 127.0.0.1

The root cause is the following:

1. htbdequeue calls htbdequeue_tree which calls the dequeue handler on the selected leaf qdisc
2. netem_dequeue calls enqueue on the child qdisc
3. blackholeenqueue drops the packet and returns a value that is not just NETXMIT_SUCCESS
4. Because of this, netemdequeue calls qdisctreereducebacklog, and since qlen is now 0, it calls htbqlennotify -> htbdeactivate -> htbdeactiviateprios -> htbremoveclassfromrow -> htbsaferberase
5. As this is the only class in the selected hprio rbtree, _rbchangechild in _rberaseaugmented sets the rb_root pointer to NULL
6. Because blackholedequeue returns NULL, netemdequeue returns NULL, which causes htbdequeuetree to call htblookupleaf with the same hprio rbtree, and fail the BUG_ON

The function graph for this scenario is shown here: 0) | htbenqueue() { 0) + 13.635 us | netemenqueue(); 0) 4.719 us | htbactivateprios(); 0) # 2249.199 us | } 0) | htbdequeue() { 0) 2.355 us | htblookupleaf(); 0) | netemdequeue() { 0) + 11.061 us | blackholeenqueue(); 0) | qdisctreereducebacklog() { 0) | qdisclookuprcu() { 0) 1.873 us | qdiscmatchfromroot(); 0) 6.292 us | } 0) 1.894 us | htbsearch(); 0) | htbqlennotify() { 0) 2.655 us | htbdeactivateprios(); 0) 6.933 us | } 0) + 25.227 us | } 0) 1.983 us | blackholedequeue(); 0) + 86.553 us | } 0) # 2932.761 us | qdiscwarnnonwc(); 0) | htblookupleaf() { 0) | BUGON();

The full original bug report can be seen here [1].

We can fix this just by returning NULL instead of the BUGON, as htbdequeuetree returns NULL when htblookup_leaf returns NULL.

[1] https://lore.kernel.org/netdev/pF5XOOIim0IuEfhI-SOxTgRvNoDwuux7UHKnE_Y5-zVd4wmGvNk2ceHjKb8ORnzw0cGwfmVu42g9dL7XyJLf1NEzaztboTWcm0Ogxuojoeo=@willsroot.io/