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

[Martin KaFai Lau]

On 8/9/23 10:12 AM, Martin KaFai Lau wrote:
> 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;

Friendly ping. Thanks.