Subject: Re: [PATCH net] sch_netem: fix skb leak in netem_enqueue()

From: Stephen Hemminger <[email-masked]>
Date: Fri, 2 Mar 2018 10:44:23 -0800

On Fri,  2 Mar 2018 21:16:48 +0300
Alexey Kodanev <[email-masked]> wrote:

> When we exceed current packets limit and have more than one
> segment in the list returned by skb_gso_segment(), netem drops
> only the first one, skipping the rest, hence kmemleak reports:
> 
> unreferenced object 0xffff880b5d23b600 (size 1024):
>   comm "softirq", pid 0, jiffies 4384527763 (age 2770.629s)
>   hex dump (first 32 bytes):
>     00 80 23 5d 0b 88 ff ff 00 00 00 00 00 00 00 00  ..#]............
>     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>   backtrace:
>     [<00000000d8a19b9d>] __alloc_skb+0xc9/0x520
>     [<000000001709b32f>] skb_segment+0x8c8/0x3710
>     [<00000000c7b9bb88>] tcp_gso_segment+0x331/0x1830
>     [<00000000c921cba1>] inet_gso_segment+0x476/0x1370
>     [<000000008b762dd4>] skb_mac_gso_segment+0x1f9/0x510
>     [<000000002182660a>] __skb_gso_segment+0x1dd/0x620
>     [<00000000412651b9>] netem_enqueue+0x1536/0x2590 [sch_netem]
>     [<0000000005d3b2a9>] __dev_queue_xmit+0x1167/0x2120
>     [<00000000fc5f7327>] ip_finish_output2+0x998/0xf00
>     [<00000000d309e9d3>] ip_output+0x1aa/0x2c0
>     [<000000007ecbd3a4>] tcp_transmit_skb+0x18db/0x3670
>     [<0000000042d2a45f>] tcp_write_xmit+0x4d4/0x58c0
>     [<0000000056a44199>] tcp_tasklet_func+0x3d9/0x540
>     [<0000000013d06d02>] tasklet_action+0x1ca/0x250
>     [<00000000fcde0b8b>] __do_softirq+0x1b4/0x5a3
>     [<00000000e7ed027c>] irq_exit+0x1e2/0x210
> 
> Fix it by adding the rest of the segments, if any, to skb
> 'to_free' list in that case.
> 
> Fixes: 6071bd1aa13e ("netem: Segment GSO packets on enqueue")
> Signed-off-by: Alexey Kodanev <[email-masked]>
> ---
>  net/sched/sch_netem.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
> index 7c179ad..a5023a2 100644
> --- a/net/sched/sch_netem.c
> +++ b/net/sched/sch_netem.c
> @@ -508,8 +508,14 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>  			1<<(prandom_u32() % 8);
>  	}
>  
> -	if (unlikely(sch->q.qlen >= sch->limit))
> +	if (unlikely(sch->q.qlen >= sch->limit)) {
> +		while (segs) {
> +			skb2 = segs->next;
> +			__qdisc_drop(segs, to_free);
> +			segs = skb2;
> +		}
>  		return qdisc_drop(skb, sch, to_free);
> +	}
>  
>  	qdisc_qstats_backlog_inc(sch, skb);
>  

Since this is a generic problem why is not fixed in qdisc_drop instead?