netdev - Re: [PATCH net v2] net: team: get rid of team->lock in team module

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Date: Tue, 19 Sep 2023 09:40:53 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Jiri Pirko <jiri@...nulli.us>, Taehee Yoo <ap420073@...il.com>
Cc: davem@...emloft.net, kuba@...nel.org, edumazet@...gle.com, 
 netdev@...r.kernel.org,
 syzbot+9bbbacfbf1e04d5221f7@...kaller.appspotmail.com, 
 syzbot+1c71587a1a09de7fbde3@...kaller.appspotmail.com
Subject: Re: [PATCH net v2] net: team: get rid of team->lock in team module

On Sat, 2023-09-16 at 18:47 +0200, Jiri Pirko wrote:
> Sat, Sep 16, 2023 at 03:11:15PM CEST, ap420073@...il.com wrote:
> > The purpose of team->lock is to protect the private data of the team
> > interface. But RTNL already protects it all well.
> > The precise purpose of the team->lock is to reduce contention of
> > RTNL due to GENL operations such as getting the team port list, and
> > configuration dump.
> > 
> > team interface has used a dynamic lockdep key to avoid false-positive
> > lockdep deadlock detection. Virtual interfaces such as team usually
> > have their own lock for protecting private data.
> > These interfaces can be nested.
> > team0
> >  |
> > team1
> > 
> > Each interface's lock is actually different(team0->lock and team1->lock).
> > So,
> > mutex_lock(&team0->lock);
> > mutex_lock(&team1->lock);
> > mutex_unlock(&team1->lock);
> > mutex_unlock(&team0->lock);
> > The above case is absolutely safe. But lockdep warns about deadlock.
> > Because the lockdep understands these two locks are same. This is a
> > false-positive lockdep warning.
> > 
> > So, in order to avoid this problem, the team interfaces started to use
> > dynamic lockdep key. The false-positive problem was fixed, but it
> > introduced a new problem.
> > 
> > When the new team virtual interface is created, it registers a dynamic
> > lockdep key(creates dynamic lockdep key) and uses it. But there is the
> > limitation of the number of lockdep keys.
> > So, If so many team interfaces are created, it consumes all lockdep keys.
> > Then, the lockdep stops to work and warns about it.
> 
> What about fixing the lockdep instead? I bet this is not the only
> occurence of this problem.

I think/fear that solving the max key lockdep problem could be
problematic hard and/or requiring an invasive change.

Is there any real use-case requiring team devices being nested one to
each other? If not, can we simply prevent such nesting in
team_port_add()? I'm guessing that syzkaller can find more ways to
exploit such complex setup.

Cheers,

Paolo