Subject: Re: [Patch net v3 1/4] pfifo_tail_enqueue: Drop new packet when sch->limit == 0

From: Simon Horman <[email-masked]>
Date: Tue, 4 Feb 2025 11:32:07 +0000

On Mon, Feb 03, 2025 at 04:58:38PM -0800, Cong Wang wrote:
> From: Quang Le <[email-masked]>
> 
> Expected behaviour:
> In case we reach scheduler's limit, pfifo_tail_enqueue() will drop a
> packet in scheduler's queue and decrease scheduler's qlen by one.
> Then, pfifo_tail_enqueue() enqueue new packet and increase
> scheduler's qlen by one. Finally, pfifo_tail_enqueue() return
> `NET_XMIT_CN` status code.
> 
> Weird behaviour:
> In case we set `sch->limit == 0` and trigger pfifo_tail_enqueue() on a
> scheduler that has no packet, the 'drop a packet' step will do nothing.
> This means the scheduler's qlen still has value equal 0.
> Then, we continue to enqueue new packet and increase scheduler's qlen by
> one. In summary, we can leverage pfifo_tail_enqueue() to increase qlen by
> one and return `NET_XMIT_CN` status code.
> 
> The problem is:
> Let's say we have two qdiscs: Qdisc_A and Qdisc_B.
>  - Qdisc_A's type must have '->graft()' function to create parent/child relationship.
>    Let's say Qdisc_A's type is `hfsc`. Enqueue packet to this qdisc will trigger `hfsc_enqueue`.
>  - Qdisc_B's type is pfifo_head_drop. Enqueue packet to this qdisc will trigger `pfifo_tail_enqueue`.
>  - Qdisc_B is configured to have `sch->limit == 0`.
>  - Qdisc_A is configured to route the enqueued's packet to Qdisc_B.
> 
> Enqueue packet through Qdisc_A will lead to:
>  - hfsc_enqueue(Qdisc_A) -> pfifo_tail_enqueue(Qdisc_B)
>  - Qdisc_B->q.qlen += 1
>  - pfifo_tail_enqueue() return `NET_XMIT_CN`
>  - hfsc_enqueue() check for `NET_XMIT_SUCCESS` and see `NET_XMIT_CN` => hfsc_enqueue() don't increase qlen of Qdisc_A.
> 
> The whole process lead to a situation where Qdisc_A->q.qlen == 0 and Qdisc_B->q.qlen == 1.
> Replace 'hfsc' with other type (for example: 'drr') still lead to the same problem.
> This violate the design where parent's qlen should equal to the sum of its childrens'qlen.
> 
> Bug impact: This issue can be used for user->kernel privilege escalation when it is reachable.
> 
> Fixes: f70f90672a2c ("sched: add head drop fifo queue")

Hi Cong,

Not a proper review, but I believe the hash in mainline for the cited
commit is 57dbb2d83d100ea.

> Reported-by: Quang Le <[email-masked]>
> Signed-off-by: Quang Le <[email-masked]>
> Signed-off-by: Cong Wang <[email-masked]>

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