Re: [PATCH] net: explicitly clear the sk pointer, when pf->create fails

[Ignat Korchagin]

On Fri, Oct 4, 2024 at 9:55 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Thu, Oct 3, 2024 at 11:50 PM Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:
> >
> > From: Ignat Korchagin <ignat@xxxxxxxxxxxxxx>
> > Date: Thu,  3 Oct 2024 18:01:51 +0100
> > > We have recently noticed the exact same KASAN splat as in commit
> > > 6cd4a78d962b ("net: do not leave a dangling sk pointer, when socket
> > > creation fails"). The problem is that commit did not fully address the
> > > problem, as some pf->create implementations do not use sk_common_release
> > > in their error paths.
> > >
> > > For example, we can use the same reproducer as in the above commit, but
> > > changing ping to arping. arping uses AF_PACKET socket and if packet_create
> > > fails, it will just sk_free the allocated sk object.
> > >
> > > While we could chase all the pf->create implementations and make sure they
> > > NULL the freed sk object on error from the socket, we can't guarantee
> > > future protocols will not make the same mistake.
> > >
> > > So it is easier to just explicitly NULL the sk pointer upon return from
> > > pf->create in __sock_create. We do know that pf->create always releases the
> > > allocated sk object on error, so if the pointer is not NULL, it is
> > > definitely dangling.
> >
> > Sounds good to me.
> >
> > Let's remove the change by 6cd4a78d962b that should be unnecessary
> > with this patch.
> >
>
> Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
>
> Even if not strictly needed we also could fix af_packet to avoid a
> dangling pointer.

af_packet was just one example - I reviewed every pf->create function
and there are others. It would not be fair to fix this, but not the
others, right?

> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index a705ec21425409dc708cf61d619545ed342b1f01..db7d3482e732f236ec461b1ff52a264d1b93bf90
> 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -3421,16 +3421,18 @@ static int packet_create(struct net *net,
> struct socket *sock, int protocol,
>         if (sock->type == SOCK_PACKET)
>                 sock->ops = &packet_ops_spkt;
>
> +       po = pkt_sk(sk);
> +       err = packet_alloc_pending(po);
> +       if (err)
> +               goto out2;
> +
> +       /* No more error after this point. */
>         sock_init_data(sock, sk);
>
> -       po = pkt_sk(sk);
>         init_completion(&po->skb_completion);
>         sk->sk_family = PF_PACKET;
>         po->num = proto;
>
> -       err = packet_alloc_pending(po);
> -       if (err)
> -               goto out2;
>
>         packet_cached_dev_reset(po);