5.4-stable review patch. If anyone has any objections, please let me know.
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From: William Liu will@willsroot.io
[ Upstream commit 0e1d5d9b5c5966e2e42e298670808590db5ed628 ]
htb_lookup_leaf 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. htb_dequeue calls htb_dequeue_tree which calls the dequeue handler on the selected leaf qdisc 2. netem_dequeue calls enqueue on the child qdisc 3. blackhole_enqueue drops the packet and returns a value that is not just NET_XMIT_SUCCESS 4. Because of this, netem_dequeue calls qdisc_tree_reduce_backlog, and since qlen is now 0, it calls htb_qlen_notify -> htb_deactivate -> htb_deactiviate_prios -> htb_remove_class_from_row -> htb_safe_rb_erase 5. As this is the only class in the selected hprio rbtree, __rb_change_child in __rb_erase_augmented sets the rb_root pointer to NULL 6. Because blackhole_dequeue returns NULL, netem_dequeue returns NULL, which causes htb_dequeue_tree to call htb_lookup_leaf with the same hprio rbtree, and fail the BUG_ON
The function graph for this scenario is shown here: 0) | htb_enqueue() { 0) + 13.635 us | netem_enqueue(); 0) 4.719 us | htb_activate_prios(); 0) # 2249.199 us | } 0) | htb_dequeue() { 0) 2.355 us | htb_lookup_leaf(); 0) | netem_dequeue() { 0) + 11.061 us | blackhole_enqueue(); 0) | qdisc_tree_reduce_backlog() { 0) | qdisc_lookup_rcu() { 0) 1.873 us | qdisc_match_from_root(); 0) 6.292 us | } 0) 1.894 us | htb_search(); 0) | htb_qlen_notify() { 0) 2.655 us | htb_deactivate_prios(); 0) 6.933 us | } 0) + 25.227 us | } 0) 1.983 us | blackhole_dequeue(); 0) + 86.553 us | } 0) # 2932.761 us | qdisc_warn_nonwc(); 0) | htb_lookup_leaf() { 0) | BUG_ON(); ------------------------------------------
The full original bug report can be seen here [1].
We can fix this just by returning NULL instead of the BUG_ON, as htb_dequeue_tree returns NULL when htb_lookup_leaf returns NULL.
[1] https://lore.kernel.org/netdev/pF5XOOIim0IuEfhI-SOxTgRvNoDwuux7UHKnE_Y5-zVd4...
Fixes: 512bb43eb542 ("pkt_sched: sch_htb: Optimize WARN_ONs in htb_dequeue_tree() etc.") Signed-off-by: William Liu will@willsroot.io Signed-off-by: Savino Dicanosa savy@syst3mfailure.io Link: https://patch.msgid.link/20250717022816.221364-1-will@willsroot.io Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/sched/sch_htb.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
--- a/net/sched/sch_htb.c +++ b/net/sched/sch_htb.c @@ -776,7 +776,9 @@ static struct htb_class *htb_lookup_leaf u32 *pid; } stk[TC_HTB_MAXDEPTH], *sp = stk;
- BUG_ON(!hprio->row.rb_node); + if (unlikely(!hprio->row.rb_node)) + return NULL; + sp->root = hprio->row.rb_node; sp->pptr = &hprio->ptr; sp->pid = &hprio->last_ptr_id;