ipv6: ioam: fix potential NULL dereferences in __ioam6_fill_trace_data()

We need to check __in6_dev_get() for possible NULL value, as
suggested by Yiming Qian.

Also add skb_dst_dev_rcu() instead of skb_dst_dev(),
and two missing READ_ONCE().

Note that @dev can't be NULL.

Fixes: 9ee11f0 ("ipv6: ioam: Data plane support for Pre-allocated Trace")
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Justin Iurman <justin.iurman@gmail.com>
Link: https://patch.msgid.link/20260402101732.1188059-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv6/ioam6.c

@@ -710,7 +710,9 @@ static void __ioam6_fill_trace_data(struct sk_buff *skb,
 				    struct ioam6_schema *sc,
 				    unsigned int sclen, bool is_input)
 {
-	struct net_device *dev = skb_dst_dev(skb);
+	/* Note: skb_dst_dev_rcu() can't be NULL at this point. */
+	struct net_device *dev = skb_dst_dev_rcu(skb);
+	struct inet6_dev *i_skb_dev, *idev;
 	struct timespec64 ts;
 	ktime_t tstamp;
 	u64 raw64;
@@ -721,32 +723,35 @@ static void __ioam6_fill_trace_data(struct sk_buff *skb,
 
 	data = trace->data + trace->remlen * 4 - trace->nodelen * 4 - sclen * 4;
 
+	i_skb_dev = skb->dev ? __in6_dev_get(skb->dev) : NULL;
+	idev = __in6_dev_get(dev);
+
 	/* hop_lim and node_id */
 	if (trace->type.bit0) {
 		byte = ipv6_hdr(skb)->hop_limit;
 		if (is_input)
 			byte--;
 
-		raw32 = dev_net(dev)->ipv6.sysctl.ioam6_id;
+		raw32 = READ_ONCE(dev_net(dev)->ipv6.sysctl.ioam6_id);
 
 		*(__be32 *)data = cpu_to_be32((byte << 24) | raw32);
 		data += sizeof(__be32);
 	}
 
 	/* ingress_if_id and egress_if_id */
 	if (trace->type.bit1) {
-		if (!skb->dev)
+		if (!i_skb_dev)
 			raw16 = IOAM6_U16_UNAVAILABLE;
 		else
-			raw16 = (__force u16)READ_ONCE(__in6_dev_get(skb->dev)->cnf.ioam6_id);
+			raw16 = (__force u16)READ_ONCE(i_skb_dev->cnf.ioam6_id);
 
 		*(__be16 *)data = cpu_to_be16(raw16);
 		data += sizeof(__be16);
 
-		if (dev->flags & IFF_LOOPBACK)
+		if ((dev->flags & IFF_LOOPBACK) || !idev)
 			raw16 = IOAM6_U16_UNAVAILABLE;
 		else
-			raw16 = (__force u16)READ_ONCE(__in6_dev_get(dev)->cnf.ioam6_id);
+			raw16 = (__force u16)READ_ONCE(idev->cnf.ioam6_id);
 
 		*(__be16 *)data = cpu_to_be16(raw16);
 		data += sizeof(__be16);
@@ -822,26 +827,26 @@ static void __ioam6_fill_trace_data(struct sk_buff *skb,
 		if (is_input)
 			byte--;
 
-		raw64 = dev_net(dev)->ipv6.sysctl.ioam6_id_wide;
+		raw64 = READ_ONCE(dev_net(dev)->ipv6.sysctl.ioam6_id_wide);
 
 		*(__be64 *)data = cpu_to_be64(((u64)byte << 56) | raw64);
 		data += sizeof(__be64);
 	}
 
 	/* ingress_if_id and egress_if_id (wide) */
 	if (trace->type.bit9) {
-		if (!skb->dev)
+		if (!i_skb_dev)
 			raw32 = IOAM6_U32_UNAVAILABLE;
 		else
-			raw32 = READ_ONCE(__in6_dev_get(skb->dev)->cnf.ioam6_id_wide);
+			raw32 = READ_ONCE(i_skb_dev->cnf.ioam6_id_wide);
 
 		*(__be32 *)data = cpu_to_be32(raw32);
 		data += sizeof(__be32);
 
-		if (dev->flags & IFF_LOOPBACK)
+		if ((dev->flags & IFF_LOOPBACK) || !idev)
 			raw32 = IOAM6_U32_UNAVAILABLE;
 		else
-			raw32 = READ_ONCE(__in6_dev_get(dev)->cnf.ioam6_id_wide);
+			raw32 = READ_ONCE(idev->cnf.ioam6_id_wide);
 
 		*(__be32 *)data = cpu_to_be32(raw32);
 		data += sizeof(__be32);