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

[Paolo Abeni <pabeni@redhat.com>]

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

Cheers,

Paolo