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

[Breno Leitao <leitao@debian.org>]

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