netdev - Re: [PATCH 9/9] udpv6: Check address length before reading address family

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Message-ID: <20190412164927.GA28199@rdna-mbp>
Date:   Fri, 12 Apr 2019 16:49:30 +0000
From:   Andrey Ignatov <rdna@...com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>,
        "David S. Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 9/9] udpv6: Check address length before reading address
 family

Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> [Fri, 2019-04-12 03:57 -0700]:
> KMSAN will complain if valid address length passed to udpv6_pre_connect()
> is shorter than sizeof("struct sockaddr"->sa_family) bytes.
> 
> (This patch is bogus if it is guaranteed that udpv6_pre_connect() is
> always called after checking "struct sockaddr"->sa_family. In that case,
> we want a comment why we don't need to check valid address length here.)
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> ---
>  net/ipv6/udp.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index d538fafaf4a9..2464fba569b4 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -1045,6 +1045,8 @@ static void udp_v6_flush_pending_frames(struct sock *sk)
>  static int udpv6_pre_connect(struct sock *sk, struct sockaddr *uaddr,
>  			     int addr_len)
>  {
> +	if (addr_len < offsetofend(struct sockaddr, sa_family))
> +		return -EINVAL;

Such a check wasn't added since it's already checked in
inet_dgram_connect, the only place where udpv6_pre_connect is called:

  int inet_dgram_connect(struct socket *sock, struct sockaddr *uaddr,
                         int addr_len, int flags)
  {
          struct sock *sk = sock->sk;
          int err;
  
          if (addr_len < sizeof(uaddr->sa_family))
                  return -EINVAL;
          if (uaddr->sa_family == AF_UNSPEC)
                  return sk->sk_prot->disconnect(sk, flags);
  
          if (BPF_CGROUP_PRE_CONNECT_ENABLED(sk)) {
                  err = sk->sk_prot->pre_connect(sk, uaddr, addr_len);
                  if (err)
                          return err;
          }

So it's already handled. But if it helps KMSAN, that's probably fine to
double-check it here. Or it's considered false positive?

>  	/* The following checks are replicated from __ip6_datagram_connect()
>  	 * and intended to prevent BPF program called below from accessing
>  	 * bytes that are out of the bound specified by user in addr_len.

-- 
Andrey Ignatov