Subject: Re: [PATCH] netlink: netlink_sendmsg: memset unused tail bytes in skb

From: Phillip Potter <[email-masked]>
Date: Tue, 11 May 2021 23:17:35 +0100

On Sun, May 09, 2021 at 03:10:51PM +0200, Florian Westphal wrote:
> Phillip Potter <[email-masked]> wrote:
> > When allocating the skb within netlink_sendmsg, with certain supplied
> > len arguments, extra bytes are allocated at the end of the data buffer,
> > due to SKB_DATA_ALIGN giving a larger size within __alloc_skb for
> > alignment reasons. This means that after using skb_put with the same
> > len value and then copying data into the skb, the skb tail area is
> > non-zero in size and contains uninitialised bytes. Wiping this area
> > (if it exists) fixes a KMSAN-found uninit-value bug reported by syzbot at:
> > https://syzkaller.appspot.com/bug?id=3e63bcec536b7136b54c72e06adeb87dc6519f69
> 
> This patch papers over the real bug.
> 
> Please fix TIPC instead.
> Incomplete patch as a starting point:
> 
> diff --git a/net/tipc/node.c b/net/tipc/node.c
> --- a/net/tipc/node.c
> +++ b/net/tipc/node.c
> @@ -2481,7 +2481,6 @@ int tipc_nl_node_get_link(struct sk_buff *skb, struct genl_info *info)
>  	struct net *net = genl_info_net(info);
>  	struct nlattr *attrs[TIPC_NLA_LINK_MAX + 1];
>  	struct tipc_nl_msg msg;
> -	char *name;
>  	int err;
>  
>  	msg.portid = info->snd_portid;
> @@ -2499,13 +2498,11 @@ int tipc_nl_node_get_link(struct sk_buff *skb, struct genl_info *info)
>  	if (!attrs[TIPC_NLA_LINK_NAME])
>  		return -EINVAL;
>  
> -	name = nla_data(attrs[TIPC_NLA_LINK_NAME]);
> -
>  	msg.skb = nlmsg_new(NLMSG_GOODSIZE, GFP_KERNEL);
>  	if (!msg.skb)
>  		return -ENOMEM;
>  
> -	if (strcmp(name, tipc_bclink_name) == 0) {
> +	if (nla_strcmp(attrs[TIPC_NLA_LINK_NAME], tipc_bclink_name) == 0) {
>  		err = tipc_nl_add_bc_link(net, &msg, tipc_net(net)->bcl);
>  		if (err)
>  			goto err_free;
> 
> 
> You will also need to change tipc_node_find_by_name() to pass the nla
> attr.
> 
> Alternatively TIPC_NLA_LINK_NAME policy can be changed:
> 
> diff --git a/net/tipc/netlink.c b/net/tipc/netlink.c
> --- a/net/tipc/netlink.c
> +++ b/net/tipc/netlink.c
> @@ -88,7 +88,7 @@ const struct nla_policy tipc_nl_net_policy[TIPC_NLA_NET_MAX + 1] = {
>  
>  const struct nla_policy tipc_nl_link_policy[TIPC_NLA_LINK_MAX + 1] = {
>         [TIPC_NLA_LINK_UNSPEC]          = { .type = NLA_UNSPEC },
> -       [TIPC_NLA_LINK_NAME]            = { .type = NLA_STRING,
> +       [TIPC_NLA_LINK_NAME]            = { .type = NLA_NUL_STRING,
> 
> 
> ... which makes it safe to treat the raw attribute payload as a c-string,
> but this might break existing userspace applications.
> 
> Its probably a good idea to audit all NLA_STRING attributes in tipc for
> similar problems.

Dear Florian,

Thank you for your feedback and code + suggestions, I will take a look at this over
the next few days and then resubmit.

Regards,
Phil