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

[Eric Dumazet]

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.

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);