netdev - Re: [PATCH net-next v6] net: ioctl: Use kernel memory on protocol ioctl callbacks

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Date: Thu, 8 Jun 2023 07:35:48 -0700
From: Breno Leitao <leitao@...ian.org>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Kuniyuki Iwashima <kuniyu@...zon.com>, alex.aring@...il.com,
	andrea.righi@...onical.com, asml.silence@...il.com, ast@...nel.org,
	axboe@...nel.dk, courmisch@...il.com, davem@...emloft.net,
	dccp@...r.kernel.org, dsahern@...nel.org, edumazet@...gle.com,
	gnault@...hat.com, hbh25y@...il.com, joannelkoong@...il.com,
	kernelxing@...cent.com, kuba@...nel.org, leit@...com,
	linux-kernel@...r.kernel.org, linux-sctp@...r.kernel.org,
	linux-wpan@...r.kernel.org, lucien.xin@...il.com,
	marcelo.leitner@...il.com, martin.lau@...nel.org,
	martineau@...nel.org, matthieu.baerts@...sares.net,
	miquel.raynal@...tlin.com, mptcp@...ts.linux.dev,
	netdev@...r.kernel.org, stefan@...enfreihafen.org,
	willemdebruijn.kernel@...il.com, wojciech.drewek@...el.com
Subject: Re: [PATCH net-next v6] net: ioctl: Use kernel memory on protocol
 ioctl callbacks

On Thu, Jun 08, 2023 at 03:57:48PM +0200, Paolo Abeni wrote:
> On Thu, 2023-06-08 at 01:43 -0700, Breno Leitao wrote:
> > Hello Kuniyuki,
> > On Wed, Jun 07, 2023 at 10:31:42AM -0700, Kuniyuki Iwashima wrote:
> > > > +/* This is the most common ioctl prep function, where the result (4 bytes) is
> > > > + * copied back to userspace if the ioctl() returns successfully. No input is
> > > > + * copied from userspace as input argument.
> > > > + */
> > > > +static int sock_ioctl_out(struct sock *sk, unsigned int cmd, void __user *arg)
> > > > +{
> > > > +	int ret, karg = 0;
> > > > +
> > > > +	ret = sk->sk_prot->ioctl(sk, cmd, &karg);
> > > 
> > > We need READ_ONCE(sk->sk_prot) as IPv4 conversion or ULP chnage could
> > > occur at the same time.
> > 
> > Thanks for the heads-up. I would like to pick you brain and understand
> > a bit more about READ_ONCE() and what is the situation that READ_ONCE()
> > will solve.
> 
> AFAICS, in this specific case READ_ONCE() should not address any "real"
> bug causing visible issue.
> 
> Still the lack of it will likely cause syzkaller report for (harmless,
> AFAICS) 'data races' around sk->sk_prot. We want to avoid such reports,
> even if harmless, because they can end-up hiding more relevant bugs.
> 
> > Is the situation related to when sock_ioctl_out() start to execute, and
> > "sk->sk_prot" changes in a different thread? If that is the case, the
> > arguments (cmd and arg) will be from the "previous" instance.
> > 
> > Also, grepping for "sk->sk_prot->", I see more than a bunch of calls
> > that do not use READ_ONCE() barrier. Why is this case different?
> 
> Races on sk->sk_prot can happen only on inet6_stream_ops (due to ulp
> and/or ADDRFORM) inet6_dgram_ops (due to ADDRFORM). AFAICS here
> READ_ONCE() is  needed as we can reach here via inet6_stream_ops-
> >inet6_ioctl

Thanks for the clarification, I will send a v6 with the READ_ONCE().

Breno