netdev - Re: [PATCH net v3 1/3] sctp: Hold RCU read lock while iterating over address list

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Message-ID: <aQJDhkIsexabyGXf@horms.kernel.org>
Date: Wed, 29 Oct 2025 16:40:38 +0000
From: Simon Horman <horms@...nel.org>
To: Stefan Wiehler <stefan.wiehler@...ia.com>
Cc: Xin Long <lucien.xin@...il.com>,
	"David S . Miller " <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Kuniyuki Iwashima <kuniyu@...gle.com>, linux-sctp@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v3 1/3] sctp: Hold RCU read lock while iterating over
 address list

On Wed, Oct 29, 2025 at 04:38:44PM +0000, Simon Horman wrote:
> On Tue, Oct 28, 2025 at 05:12:26PM +0100, Stefan Wiehler wrote:
> > With CONFIG_PROVE_RCU_LIST=y and by executing
> > 
> >   $ netcat -l --sctp &
> >   $ netcat --sctp localhost &
> >   $ ss --sctp
> > 
> > one can trigger the following Lockdep-RCU splat(s):
> 
> ...
> 
> > diff --git a/net/sctp/diag.c b/net/sctp/diag.c
> > index 996c2018f0e6..1a8761f87bf1 100644
> > --- a/net/sctp/diag.c
> > +++ b/net/sctp/diag.c
> > @@ -73,19 +73,23 @@ static int inet_diag_msg_sctpladdrs_fill(struct sk_buff *skb,
> >  	struct nlattr *attr;
> >  	void *info = NULL;
> >  
> > +	rcu_read_lock();
> >  	list_for_each_entry_rcu(laddr, address_list, list)
> >  		addrcnt++;
> > +	rcu_read_unlock();
> >  
> >  	attr = nla_reserve(skb, INET_DIAG_LOCALS, addrlen * addrcnt);
> >  	if (!attr)
> >  		return -EMSGSIZE;
> >  
> >  	info = nla_data(attr);
> 
> Hi Stefan,
> 
> If the number of entries in list increases while rcu_read_lock is not held,
> between when addrcnt is calculated and when info is written, then can an
> overrun occur while writing info?

Oops, I now see that is addressed in patch 2/3.
Sorry for not reading that before sending my previous email.

> 
> > +	rcu_read_lock();
> >  	list_for_each_entry_rcu(laddr, address_list, list) {
> >  		memcpy(info, &laddr->a, sizeof(laddr->a));
> >  		memset(info + sizeof(laddr->a), 0, addrlen - sizeof(laddr->a));
> >  		info += addrlen;
> >  	}
> > +	rcu_read_unlock();
> >  
> >  	return 0;
> >  }
> > -- 
> > 2.51.0
> >