linux-kernel - Re: [PATCH net v2] net: sched: ematch: reject invalid data

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Message-ID: <Y542i4NHrt7RIA+C@pop-os.localdomain>
Date:   Sat, 17 Dec 2022 13:37:15 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Jun Nie <jun.nie@...aro.org>, jhs@...atatu.com, jiri@...nulli.us,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2] net: sched: ematch: reject invalid data

On Thu, Dec 15, 2022 at 01:50:43PM +0100, Paolo Abeni wrote:
> On Wed, 2022-12-14 at 10:20 +0800, Jun Nie wrote:
> > ---
> >  net/sched/em_cmp.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/sched/em_cmp.c b/net/sched/em_cmp.c
> > index f17b049ea530..0284394be53f 100644
> > --- a/net/sched/em_cmp.c
> > +++ b/net/sched/em_cmp.c
> > @@ -22,9 +22,14 @@ static int em_cmp_match(struct sk_buff *skb, struct tcf_ematch *em,
> >  			struct tcf_pkt_info *info)
> >  {
> >  	struct tcf_em_cmp *cmp = (struct tcf_em_cmp *) em->data;
> > -	unsigned char *ptr = tcf_get_base_ptr(skb, cmp->layer) + cmp->off;
> > +	unsigned char *ptr;
> >  	u32 val = 0;
> >  
> > +	if (!cmp)
> > +		return 0;
> 
> It feels like this is papering over the real issue. Why em->data is
> NULL here? why other ematches are not afflicted by this issue? 
> 
> is em->data really NULL or some small value instead? KASAN seams to
> tell it's a small value, not 0, so this patch should not avoid the
> oops. Have you tested it vs the reproducer?

Right. I think I have found the root cause, let me test my patch to see
if it makes syzbot happy.

Thanks.