netdev - Re: [PATCH net-next v2 3/3] netlink: Fix wraparound of sk->sk_rmem

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Message-ID: <20250624150357.247c9468@kernel.org>
Date: Tue, 24 Jun 2025 15:03:57 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Kuniyuki Iwashima <kuni1840@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, horms@...nel.org,
 jbaron@...mai.com, kuniyu@...gle.com, netdev@...r.kernel.org,
 pabeni@...hat.com
Subject: Re: [PATCH net-next v2 3/3] netlink: Fix wraparound of
 sk->sk_rmem_alloc

On Tue, 24 Jun 2025 10:08:41 -0700 Kuniyuki Iwashima wrote:
> Date: Tue, 24 Jun 2025 07:11:57 -0700
> From: Jakub Kicinski <kuba@...nel.org>
> > On Tue, 24 Jun 2025 09:55:15 +0200 Paolo Abeni wrote:  
> > > > To be clear -- are you saying we should fix this differently?
> > > > Or perhaps that the problem doesn't exist? The change doesn't
> > > > seem very intrusive..    
> > > 
> > > AFAICS the race is possible even with netlink as netlink_unicast() runs
> > > without the socket lock, too.
> > > 
> > > The point is that for UDP the scenario with multiple threads enqueuing a
> > > packet into the same socket is a critical path, optimizing for
> > > performances and allowing some memory accounting inaccuracy makes sense.
> > > 
> > > For netlink socket, that scenario looks a patological one and I think we
> > > should prefer accuracy instead of optimization.  
> > 
> > Could you ELI5 what you mean? Are you suggesting a lock around every
> > sk_rmem write for netlink sockets? 
> > If we think this is an attack vector the attacker can simply use a UDP
> > socket instead. Or do you think it'd lead to simpler code?  
> 
> I was wondering if atomic_add_return() is expensive for netlink,
> and if not, we could use it like below. 

Ah, got it. That does look simpler. 

nit: Please don't hide the atomic_add_return() in local variable init,
as it need validation and error handling.

> I'm also not sure we want to keep the allow-at-least-one-skb rule for
> netlink though, which comes from the first condition in
> __sock_queue_rcv_skb() for UDP in the past, IIRC.