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Message-ID: <5bd6ced877e97ac674d1308eab0b8d2107b7ab85.camel@redhat.com>
Subject: Re: [PATCH net-next v6] net: ioctl: Use kernel memory on protocol
 ioctl callbacks
From:   Paolo Abeni <pabeni@redhat.com>
To:     Breno Leitao <leitao@debian.org>,
        Kuniyuki Iwashima <kuniyu@amazon.com>
Cc:     alex.aring@gmail.com, andrea.righi@canonical.com,
        asml.silence@gmail.com, ast@kernel.org, axboe@kernel.dk,
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        lucien.xin@gmail.com, marcelo.leitner@gmail.com,
        martin.lau@kernel.org, martineau@kernel.org,
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        wojciech.drewek@intel.com
Date:   Thu, 08 Jun 2023 15:57:48 +0200
In-Reply-To: <ZIGUofpP4k24qfQs@gmail.com>
References: <20230606180045.827659-1-leitao@debian.org>
         <20230607173142.86395-1-kuniyu@amazon.com> <ZIGUofpP4k24qfQs@gmail.com>
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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 =3D 0;
> > > +
> > > +	ret =3D sk->sk_prot->ioctl(sk, cmd, &karg);
> >=20
> > We need READ_ONCE(sk->sk_prot) as IPv4 conversion or ULP chnage could
> > occur at the same time.
>=20
> 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.
>=20
> 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

Cheers,

Paolo