netdev - Re: [PATCH bpf 2/2] bpf: net: Avoid incorrect bpf_sk_reuseport

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Date:   Tue, 7 Jul 2020 22:45:30 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Networking <netdev@...r.kernel.org>,
        James Chapman <jchapman@...alix.com>
Subject: Re: [PATCH bpf 2/2] bpf: net: Avoid incorrect bpf_sk_reuseport_detach call

On Tue, Jul 7, 2020 at 6:46 PM Martin KaFai Lau <kafai@...com> wrote:
>
> bpf_sk_reuseport_detach is currently called when sk->sk_user_data
> is not NULL.  It is incorrect because sk->sk_user_data may not be
> managed by the bpf's reuseport_array.  It has been report in [1] that,
> the bpf_sk_reuseport_detach() which is called from udp_lib_unhash() has
> corrupted the sk_user_data managed by l2tp.
>
> This patch solves it by using another bit (defined as SK_USER_DATA_BPF)
> of the sk_user_data pointer value.  It marks that a sk_user_data is
> managed/owned by BPF.
>
> The patch depends on a PTRMASK introduced in
> commit f1ff5ce2cd5e ("net, sk_msg: Clear sk_user_data pointer on clone if tagged").
>
> [ Note: sk->sk_user_data is used by bpf's reuseport_array only when a sk is
>   added to the bpf's reuseport_array.
>   i.e. doing setsockopt(SO_REUSEPORT) and having "sk->sk_reuseport == 1"
>   alone will not stop sk->sk_user_data being used by other means. ]
>
> [1]: https://lore.kernel.org/netdev/20200706121259.GA20199@katalix.com/
>
> Reported-by: James Chapman <jchapman@...alix.com>
> Cc: James Chapman <jchapman@...alix.com>
> Fixes: 5dc4c4b7d4e8 ("bpf: Introduce BPF_MAP_TYPE_REUSEPORT_SOCKARRAY")
> Signed-off-by: Martin KaFai Lau <kafai@...com>
> ---
>  include/net/sock.h           | 3 ++-
>  kernel/bpf/reuseport_array.c | 5 +++--
>  2 files changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 3428619faae4..9fe42c890706 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -533,7 +533,8 @@ enum sk_pacing {
>   * be copied.
>   */
>  #define SK_USER_DATA_NOCOPY    1UL
> -#define SK_USER_DATA_PTRMASK   ~(SK_USER_DATA_NOCOPY)
> +#define SK_USER_DATA_BPF       2UL     /* Managed by BPF */
> +#define SK_USER_DATA_PTRMASK   ~3UL

nit: ~3UL looks like a random constant, while

~(SK_USER_DATA_NOCOPY | SK_USER_DATA_BPF)

would clearly indicate what's going on. Original PTRMASK definition
had this logical connection with NOCOPY bit, I think it's worth it to
preserve that.

>
>  /**
>   * sk_user_data_is_nocopy - Test if sk_user_data pointer must not be copied
> diff --git a/kernel/bpf/reuseport_array.c b/kernel/bpf/reuseport_array.c
> index a95bc8d7e812..cae9d505e04a 100644
> --- a/kernel/bpf/reuseport_array.c
> +++ b/kernel/bpf/reuseport_array.c
> @@ -24,7 +24,7 @@ void bpf_sk_reuseport_detach(struct sock *sk)
>
>         write_lock_bh(&sk->sk_callback_lock);
>         sk_user_data = (uintptr_t)sk->sk_user_data;
> -       if (sk_user_data) {
> +       if (sk_user_data & SK_USER_DATA_BPF) {
>                 struct sock __rcu **socks;
>
>                 socks = (void *)(sk_user_data & SK_USER_DATA_PTRMASK);
> @@ -309,7 +309,8 @@ int bpf_fd_reuseport_array_update_elem(struct bpf_map *map, void *key,
>         if (err)
>                 goto put_file_unlock;
>
> -       sk_user_data = (uintptr_t)&array->ptrs[index] | SK_USER_DATA_NOCOPY;
> +       sk_user_data = (uintptr_t)&array->ptrs[index] | SK_USER_DATA_NOCOPY |
> +               SK_USER_DATA_BPF;
>         WRITE_ONCE(nsk->sk_user_data, (void *)sk_user_data);
>         rcu_assign_pointer(array->ptrs[index], nsk);
>         free_osk = osk;
> --
> 2.24.1
>