[PATCH net] net: use skb_to_full_sk() in skb_update_prio()

[Eric Dumazet]

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: ca6fb0651883 ("tcp: attach SYNACK messages to request sockets instead of 
listener")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Reported-by: Andrei Vagin <ava...@virtuozzo.com>
---
 include/linux/cgroup-defs.h |  4 ++--
 net/core/dev.c              | 22 +++++++++++++++-------
 2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 
9f242b876fde2c29b1d3c023a3934ef01d04cfa9..f8e76d01a5ade411c231c926e55e59a71dde6cc5
 100644
--- a/include/linux/cgroup-defs.h
+++ b/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;
diff --git a/net/core/dev.c b/net/core/dev.c
index 
2cedf520cb28f0c5255b95d0cef536ea21758672..12be205357146f0dcd55cc6e6f71dfb65fdeb33b
 100644
--- a/net/core/dev.c
+++ b/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)
-- 
2.16.2.804.g6dcf76e118-goog