netdev - Re: [PATCH bpf 1/2] bpf: syzkaller found null ptr deref in unix

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Date: Fri, 1 Dec 2023 10:24:53 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: John Fastabend <john.fastabend@...il.com>
Cc: kuniyu@...zon.com, jakub@...udflare.com, bpf@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH bpf 1/2] bpf: syzkaller found null ptr deref in unix_bpf
 proto add

On Fri, Dec 1, 2023 at 4:23 AM John Fastabend <john.fastabend@...il.com> wrote:
>
> I added logic to track the sock pair for stream_unix sockets so that we
> ensure lifetime of the sock matches the time a sockmap could reference
> the sock (see fixes tag). I forgot though that we allow af_unix unconnected
> sockets into a sock{map|hash} map.
>
> This is problematic because previous fixed expected sk_pair() to exist
> and did not NULL check it. Because unconnected sockets have a NULL
> sk_pair this resulted in the NULL ptr dereference found by syzkaller.
>
> BUG: KASAN: null-ptr-deref in unix_stream_bpf_update_proto+0x72/0x430 net/unix/unix_bpf.c:171
> Write of size 4 at addr 0000000000000080 by task syz-executor360/5073
> Call Trace:
>  <TASK>
>  ...
>  sock_hold include/net/sock.h:777 [inline]
>  unix_stream_bpf_update_proto+0x72/0x430 net/unix/unix_bpf.c:171
>  sock_map_init_proto net/core/sock_map.c:190 [inline]
>  sock_map_link+0xb87/0x1100 net/core/sock_map.c:294
>  sock_map_update_common+0xf6/0x870 net/core/sock_map.c:483
>  sock_map_update_elem_sys+0x5b6/0x640 net/core/sock_map.c:577
>  bpf_map_update_value+0x3af/0x820 kernel/bpf/syscall.c:167
>
> We considered just checking for the null ptr and skipping taking a ref
> on the NULL peer sock. But, if the socket is then connected() after
> being added to the sockmap we can cause the original issue again. So
> instead this patch blocks adding af_unix sockets that are not in the
> ESTABLISHED state.


This (and the name chosen for sk_is_unix() helper) is a bit confusing ?

When you say "af_unix sockets" you seem to imply STREAM sockets.


>
> Reported-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: syzbot+e8030702aefd3444fb9e@...kaller.appspotmail.com
> Fixes: 8866730aed51 ("bpf, sockmap: af_unix stream sockets need to hold ref for pair sock")
> Signed-off-by: John Fastabend <john.fastabend@...il.com>
> ---
>  include/net/sock.h  | 5 +++++
>  net/core/sock_map.c | 2 ++
>  2 files changed, 7 insertions(+)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 1d6931caf0c3..ea1155d68f0b 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -2799,6 +2799,11 @@ static inline bool sk_is_tcp(const struct sock *sk)
>         return sk->sk_type == SOCK_STREAM && sk->sk_protocol == IPPROTO_TCP;
>  }
>
> +static inline bool sk_is_unix(const struct sock *sk)

Maybe sk_is_stream_unix() ?

> +{
> +       return sk->sk_family == AF_UNIX && sk->sk_type == SOCK_STREAM;
> +}
> +
>  /**
>   * sk_eat_skb - Release a skb if it is no longer needed
>   * @sk: socket to eat this skb from
> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index 4292c2ed1828..448aea066942 100644
> --- a/net/core/sock_map.c
> +++ b/net/core/sock_map.c
> @@ -536,6 +536,8 @@ static bool sock_map_sk_state_allowed(const struct sock *sk)
>  {
>         if (sk_is_tcp(sk))
>                 return (1 << sk->sk_state) & (TCPF_ESTABLISHED | TCPF_LISTEN);
> +       if (sk_is_unix(sk))
> +               return (1 << sk->sk_state) & TCPF_ESTABLISHED;
>         return true;
>  }
>
> --
> 2.33.0
>