Subject: Re: [syzbot] upstream boot error: WARNING in netlink_ack

From: Jakub Kicinski <[email-masked]>
Date: Tue, 4 Oct 2022 10:42:53 -0700

On Tue, 04 Oct 2022 07:36:55 -0700 Kees Cook wrote:
> This is fixed in the pending netdev tree coming for the merge window.

This has been weighing on my conscience a little, I don't like how we
still depend on putting one length in the skb and then using a
different one for the actual memcpy(). How would you feel about this
patch on top (untested):

diff --git a/include/net/netlink.h b/include/net/netlink.h
index 4418b1981e31..6ad671441dff 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -931,6 +931,29 @@ static inline struct nlmsghdr *nlmsg_put(struct sk_buff *skb, u32 portid, u32 se
 	return __nlmsg_put(skb, portid, seq, type, payload, flags);
 }
 
+/**
+ * nlmsg_append - Add more data to a nlmsg in a skb
+ * @skb: socket buffer to store message in
+ * @nlh: message header
+ * @payload: length of message payload
+ *
+ * Append data to an existing nlmsg, used when constructing a message
+ * with multiple fixed-format headers (which is rare).
+ * Returns NULL if the tailroom of the skb is insufficient to store
+ * the extra payload.
+ */
+static inline void *nlmsg_append(struct sk_buff *skb, struct nlmsghdr *nlh,
+				 u32 size)
+{
+	if (unlikely(skb_tailroom(skb) < NLMSG_ALIGN(size)))
+		return NULL;
+
+	if (!__builtin_constant_p(size) || NLMSG_ALIGN(size) - size != 0)
+		memset(skb_tail_pointer(skb) + size, 0,
+		       NLMSG_ALIGN(size) - size);
+	return __skb_put(NLMSG_ALIGN(size));
+}
+
 /**
  * nlmsg_put_answer - Add a new callback based netlink message to an skb
  * @skb: socket buffer to store message in
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index a662e8a5ff84..bb3d855d1f57 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2488,19 +2488,28 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 		flags |= NLM_F_ACK_TLVS;
 
 	skb = nlmsg_new(payload + tlvlen, GFP_KERNEL);
-	if (!skb) {
-		NETLINK_CB(in_skb).sk->sk_err = ENOBUFS;
-		sk_error_report(NETLINK_CB(in_skb).sk);
-		return;
-	}
+	if (!skb)
+		goto err_bad_put;
 
 	rep = nlmsg_put(skb, NETLINK_CB(in_skb).portid, nlh->nlmsg_seq,
-			NLMSG_ERROR, payload, flags);
+			NLMSG_ERROR, sizeof(*errmsg), flags);
+	if (!rep)
+		goto err_bad_put;
 	errmsg = nlmsg_data(rep);
 	errmsg->error = err;
-	unsafe_memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg)
-					 ? nlh->nlmsg_len : sizeof(*nlh),
-		      /* Bounds checked by the skb layer. */);
+	memcpy(&errmsg->msg, nlh, sizeof(*nlh));
+
+	if (!(flags & NLM_F_CAPPED)) {
+		size_t data_len = nlh->nlmsg_len - sizeof(*nlh);
+		void *data;
+
+		data = nlmsg_append(skb, rep, data_len);
+		if (!data)
+			goto err_bad_put;
+
+		/* the nlh + 1 is probably going to make you unhappy? */
+		memcpy(data, nlh + 1, data_len);
+	}
 
 	if (tlvlen)
 		netlink_ack_tlv_fill(in_skb, skb, nlh, err, extack);
@@ -2508,6 +2517,12 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 	nlmsg_end(skb, rep);
 
 	nlmsg_unicast(in_skb->sk, skb, NETLINK_CB(in_skb).portid);
+
+	return;
+
+err_bad_put:
+	NETLINK_CB(in_skb).sk->sk_err = ENOBUFS;
+	sk_error_report(NETLINK_CB(in_skb).sk);
 }
 EXPORT_SYMBOL(netlink_ack);