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Date:   Mon, 29 Apr 2019 09:03:03 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Willem de Bruijn' <willemdebruijn.kernel@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "ebiederm@...ssion.com" <ebiederm@...ssion.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: RE: [PATCH net] packet: in recvmsg msg_name return at least
 sockaddr_ll

From: Willem de Bruijn [mailto:willemdebruijn.kernel@...il.com]
> Sent: 26 April 2019 20:30
> Packet send checks that msg_name is at least sizeof sockaddr_ll.
> Packet recv must return at least this length, so that its output
> can be passed unmodified to packet send.
> 
> This ceased to be true since adding support for lladdr longer than
> sll_addr. Since, the return value uses true address length.
> 
> Always return at least sizeof sockaddr_ll, even if address length
> is shorter. Zero the padding bytes.
> 
> Fixes: 0fb375fb9b93 ("[AF_PACKET]: Allow for > 8 byte hardware addresses.")
> Suggested-by: David Laight <David.Laight@...lab.com>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> ---
>  net/packet/af_packet.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 13301e36b4a28..ca38e75c702e7 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -3358,9 +3358,14 @@ static int packet_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
>  			msg->msg_namelen = sizeof(struct sockaddr_pkt);
>  		} else {
>  			struct sockaddr_ll *sll = &PACKET_SKB_CB(skb)->sa.ll;
> -
>  			msg->msg_namelen = sll->sll_halen +
>  				offsetof(struct sockaddr_ll, sll_addr);
> +			if (msg->msg_namelen < sizeof(struct sockaddr_ll)) {
> +				memset(msg->msg_name +
> +				       offsetof(struct sockaddr_ll, sll_addr),
> +				       0, sizeof(sll->sll_addr));
> +				msg->msg_namelen = sizeof(struct sockaddr_ll);
> +			}
>  		}
>  		memcpy(msg->msg_name, &PACKET_SKB_CB(skb)->sa,
>  		       msg->msg_namelen);

That memcpy() carefully overwrites the zeroed bytes.
You need a separate 'copy_len' that isn't updated (from 18 to 20).

	David

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