Re: [PATCH] netlink: netlink_sendmsg: memset unused tail bytes in skb
From: Florian Westphal <fw@strlen.de>
Date: 2021-05-09 13:11:14
Also in:
lkml
Subsystem:
networking [general], the rest, tipc network layer · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds, Jon Maloy
Phillip Potter [off-list ref] 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.