af_key: unconditionally clone on broadcast

Attempting to avoid cloning the skb when broadcasting by inflating
the refcount with sock_hold/sock_put while under RCU lock is dangerous
and violates RCU principles. It leads to subtle race conditions when
attempting to free the SKB, as we may reference sockets that have
already been freed by the stack.

Unable to handle kernel paging request at virtual address 6b6b6b6b6b6c4b
[006b6b6b6b6b6c4b] address between user and kernel address ranges
Internal error: Oops: 96000004 [#1] PREEMPT SMP
task: fffffff78f65b380 task.stack: ffffff8049a88000
pc : sock_rfree+0x38/0x6c
lr : skb_release_head_state+0x6c/0xcc
Process repro (pid: 7117, stack limit = 0xffffff8049a88000)
Call trace:
sock_rfree+0x38/0x6c
skb_release_head_state+0x6c/0xcc
skb_release_all+0x1c/0x38
__kfree_skb+0x1c/0x30
kfree_skb+0xd0/0xf4
pfkey_broadcast+0x14c/0x18c
pfkey_sendmsg+0x1d8/0x408
sock_sendmsg+0x44/0x60
___sys_sendmsg+0x1d0/0x2a8
__sys_sendmsg+0x64/0xb4
SyS_sendmsg+0x34/0x4c
el0_svc_naked+0x34/0x38
Kernel panic - not syncing: Fatal exception

Suggested-by: Eric Dumazet <[e-mail]>
Signed-off-by: Sean Tranchetti <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

net/key/af_key.c

···
 	return 0;
 }
 
-static int pfkey_broadcast_one(struct sk_buff *skb, struct sk_buff **skb2,
-			       gfp_t allocation, struct sock *sk)
+static int pfkey_broadcast_one(struct sk_buff *skb, gfp_t allocation,
+			       struct sock *sk)
 {
 	int err = -ENOBUFS;
 
-	sock_hold(sk);
-	if (*skb2 == NULL) {
-		if (refcount_read(&skb->users) != 1) {
-			*skb2 = skb_clone(skb, allocation);
-		} else {
-			*skb2 = skb;
-			refcount_inc(&skb->users);
-		}
+	if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf)
+		return err;
+
+	skb = skb_clone(skb, allocation);
+
+	if (skb) {
+		skb_set_owner_r(skb, sk);
+		skb_queue_tail(&sk->sk_receive_queue, skb);
+		sk->sk_data_ready(sk);
+		err = 0;
 	}
-	if (*skb2 != NULL) {
-		if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf) {
-			skb_set_owner_r(*skb2, sk);
-			skb_queue_tail(&sk->sk_receive_queue, *skb2);
-			sk->sk_data_ready(sk);
-			*skb2 = NULL;
-			err = 0;
-		}
-	}
-	sock_put(sk);
 	return err;
 }
 
···
 {
 	struct netns_pfkey *net_pfkey = net_generic(net, pfkey_net_id);
 	struct sock *sk;
-	struct sk_buff *skb2 = NULL;
 	int err = -ESRCH;
 
 	/* XXX Do we need something like netlink_overrun?  I think
···
 		 * socket.
 		 */
 		if (pfk->promisc)
-			pfkey_broadcast_one(skb, &skb2, GFP_ATOMIC, sk);
+			pfkey_broadcast_one(skb, GFP_ATOMIC, sk);
 
 		/* the exact target will be processed later */
 		if (sk == one_sk)
···
 				continue;
 		}
 
-		err2 = pfkey_broadcast_one(skb, &skb2, GFP_ATOMIC, sk);
+		err2 = pfkey_broadcast_one(skb, GFP_ATOMIC, sk);
 
 		/* Error is cleared after successful sending to at least one
 		 * registered KM */
···
 	rcu_read_unlock();
 
 	if (one_sk != NULL)
-		err = pfkey_broadcast_one(skb, &skb2, allocation, one_sk);
+		err = pfkey_broadcast_one(skb, allocation, one_sk);
 
-	kfree_skb(skb2);
 	kfree_skb(skb);
 	return err;
 }