net: use skb_to_full_sk() in skb_update_prio()

Andrei Vagin reported a KASAN: slab-out-of-bounds error in
skb_update_prio()

Since SYNACK might be attached to a request socket, we need to
get back to the listener socket.
Since this listener is manipulated without locks, add const
qualifiers to sock_cgroup_prioidx() so that the const can also
be used in skb_update_prio()

Also add the const qualifier to sock_cgroup_classid() for consistency.

Fixes: ca6fb065 ("tcp: attach SYNACK messages to request sockets instead of listener")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: NAndrei Vagin <avagin@virtuozzo.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/cgroup-defs.h

@@ -755,13 +755,13 @@ struct sock_cgroup_data {
  * updaters and return part of the previous pointer as the prioidx or
  * classid.  Such races are short-lived and the result isn't critical.
  */
-static inline u16 sock_cgroup_prioidx(struct sock_cgroup_data *skcd)
+static inline u16 sock_cgroup_prioidx(const struct sock_cgroup_data *skcd)
 {
 	/* fallback to 1 which is always the ID of the root cgroup */
 	return (skcd->is_data & 1) ? skcd->prioidx : 1;
 }
 
-static inline u32 sock_cgroup_classid(struct sock_cgroup_data *skcd)
+static inline u32 sock_cgroup_classid(const struct sock_cgroup_data *skcd)
 {
 	/* fallback to 0 which is the unconfigured default classid */
 	return (skcd->is_data & 1) ? skcd->classid : 0;

net/core/dev.c

@@ -3278,15 +3278,23 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 #if IS_ENABLED(CONFIG_CGROUP_NET_PRIO)
 static void skb_update_prio(struct sk_buff *skb)
 {
-	struct netprio_map *map = rcu_dereference_bh(skb->dev->priomap);
+	const struct netprio_map *map;
+	const struct sock *sk;
+	unsigned int prioidx;
 
-	if (!skb->priority && skb->sk && map) {
-		unsigned int prioidx =
-			sock_cgroup_prioidx(&skb->sk->sk_cgrp_data);
+	if (skb->priority)
+		return;
+	map = rcu_dereference_bh(skb->dev->priomap);
+	if (!map)
+		return;
+	sk = skb_to_full_sk(skb);
+	if (!sk)
+		return;
 
-		if (prioidx < map->priomap_len)
-			skb->priority = map->priomap[prioidx];
-	}
+	prioidx = sock_cgroup_prioidx(&sk->sk_cgrp_data);
+
+	if (prioidx < map->priomap_len)
+		skb->priority = map->priomap[prioidx];
 }
 #else
 #define skb_update_prio(skb)