Re: [PATCH bpf-next] net: Fix slab-out-of-bounds in inet[6]_steal_sock

[Martin KaFai Lau]

On 8/9/23 8:55 AM, Kuniyuki Iwashima wrote:
> From: Lorenz Bauer <lmb@xxxxxxxxxxxxx>
> Date: Wed, 9 Aug 2023 16:08:31 +0100
> > On Wed, Aug 9, 2023 at 3:39 PM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
> > > On 8/9/23 1:33 AM, Lorenz Bauer wrote:
> > > > Kumar reported a KASAN splat in tcp_v6_rcv:
> > > >
> > > >     bash-5.2# ./test_progs -t btf_skc_cls_ingress
> > > >     ...
> > > >     [   51.810085] BUG: KASAN: slab-out-of-bounds in tcp_v6_rcv+0x2d7d/0x3440
> > > >     [   51.810458] Read of size 2 at addr ffff8881053f038c by task test_progs/226
> > > >
> > > > The problem is that inet[6]_steal_sock accesses sk->sk_protocol without
> > > > accounting for request sockets. I added the check to ensure that we only
> > > > every try to perform a reuseport lookup on a supported socket.
> > > >
> > > > It turns out that this isn't necessary at all. struct sock_common contains
> > > > a skc_reuseport flag which indicates whether a socket is part of a
> > >
> > > Does it go back to the earlier discussion
> > > (https://lore.kernel.org/bpf/7188429a-c380-14c8-57bb-9d05d3ba4e5e@xxxxxxxxx/)
> > > that the sk->sk_reuseport is 1 from sk_clone for TCP_ESTABLISHED? It works
> > > because there is sk->sk_reuseport"_cb" check going deeper into
> > > reuseport_select_sock() but there is an extra inet6_ehashfn for all TCP_ESTABLISHED.
> >
> > Sigh, I'd forgotten about this...
> >
> > For the TPROXY TCP replacement use case we sk_assign the SYN to the
> > listener, which creates the reqsk. We can let follow up packets pass
> > without sk_assign since they will match the reqsk and convert to a
> > fullsock via the usual route. At least that is what the test does. I'm
> > not even sure what it means to redirect a random packet into an
> > established TCP socket TBH. It'd probably be dropped?

It could act like an earlier early-demux for established sk? If the bpf prog has 
already looked up an established sk for other needs (eg. reading the sk local 
storage), it may as well bpf_sk_assign it to the skb. I don't have a use case 
for that but I also don't see why it won't work also.

> > For UDP, I'm not sure whether we even get into this situation? Doesn't
> > seem like UDP sockets are cloned from each other, so we also shouldn't
> > end up with a reuseport flag set erroneously.
> >
> > Things we could do if necessary:
> > 1. Reset the flag in inet_csk_clone_lock like we do for SOCK_RCU_FREE
>
> I think we can't do this as sk_reuseport is inherited to twsk and used
> in inet_bind_conflict().
>
>
> > 2. Duplicate the cb check into inet[6]_steal_sock
>
> or 3. Add sk_fullsock() test ?

yeah, probably adding sk_fullsock() is needed, may be something like(?):

	if (!prefetched || !sk_fullsock(sk))
                return sk;