netdev - Re: [PATCH net] net: bpf: fix request

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Date:   Thu, 9 Jun 2022 15:22:10 -0700
From:   Joe Stringer <joe@...ium.io>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Jon Maxwell <jmaxwell37@...il.com>, netdev@...r.kernel.org,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, atenart@...nel.org, cutaylor-pub@...oo.com,
        alexei.starovoitov@...il.com, kafai@...com, i@....io,
        bpf@...r.kernel.org
Subject: Re: [PATCH net] net: bpf: fix request_sock leak in filter.c

On Thu, Jun 9, 2022 at 1:30 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 6/9/22 3:18 AM, Jon Maxwell wrote:
> > A customer reported a request_socket leak in a Calico cloud environment. We
> > found that a BPF program was doing a socket lookup with takes a refcnt on
> > the socket and that it was finding the request_socket but returning the parent
> > LISTEN socket via sk_to_full_sk() without decrementing the child request socket
> > 1st, resulting in request_sock slab object leak. This patch retains the
> > existing behaviour of returning full socks to the caller but it also decrements
> > the child request_socket if one is present before doing so to prevent the leak.
> >
> > Thanks to Curtis Taylor for all the help in diagnosing and testing this. And
> > thanks to Antoine Tenart for the reproducer and patch input.
> >
> > Fixes: f7355a6c0497 bpf: ("Check sk_fullsock() before returning from bpf_sk_lookup()")
> > Fixes: edbf8c01de5a bpf: ("add skc_lookup_tcp helper")
> > Tested-by: Curtis Taylor <cutaylor-pub@...oo.com>
> > Co-developed-by: Antoine Tenart <atenart@...nel.org>
> > Signed-off-by:: Antoine Tenart <atenart@...nel.org>
> > Signed-off-by: Jon Maxwell <jmaxwell37@...il.com>
> > ---
> >   net/core/filter.c | 20 ++++++++++++++------
> >   1 file changed, 14 insertions(+), 6 deletions(-)
> >
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 2e32cee2c469..e3c04ae7381f 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -6202,13 +6202,17 @@ __bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
> >   {
> >       struct sock *sk = __bpf_skc_lookup(skb, tuple, len, caller_net,
> >                                          ifindex, proto, netns_id, flags);
> > +     struct sock *sk1 = sk;
> >
> >       if (sk) {
> >               sk = sk_to_full_sk(sk);
> > -             if (!sk_fullsock(sk)) {
> > -                     sock_gen_put(sk);
> > +             /* sk_to_full_sk() may return (sk)->rsk_listener, so make sure the original sk1
> > +              * sock refcnt is decremented to prevent a request_sock leak.
> > +              */
> > +             if (!sk_fullsock(sk1))
> > +                     sock_gen_put(sk1);
> > +             if (!sk_fullsock(sk))
> >                       return NULL;
>
> [ +Martin/Joe/Lorenz ]
>
> I wonder, should we also add some asserts in here to ensure we don't get an unbalance for the
> bpf_sk_release() case later on? Rough pseudocode could be something like below:
>
> static struct sock *
> __bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
>                  struct net *caller_net, u32 ifindex, u8 proto, u64 netns_id,
>                  u64 flags)
> {
>          struct sock *sk = __bpf_skc_lookup(skb, tuple, len, caller_net,
>                                             ifindex, proto, netns_id, flags);
>          if (sk) {
>                  struct sock *sk2 = sk_to_full_sk(sk);
>
>                  if (!sk_fullsock(sk2))
>                          sk2 = NULL;
>                  if (sk2 != sk) {
>                          sock_gen_put(sk);
>                          if (unlikely(sk2 && !sock_flag(sk2, SOCK_RCU_FREE))) {
>                                  WARN_ONCE(1, "Found non-RCU, unreferenced socket!");
>                                  sk2 = NULL;
>                          }
>                  }
>                  sk = sk2;
>          }
>          return sk;
> }

This seems a bit more readable to me from the perspective of
understanding the way that the socket references are tracked & freed.