net: Add locking to protect skb->dev access in ip_output

In ip_output() skb->dev is updated from the skb_dst(skb)->dev
this can become invalid when the interface is unregistered and freed,

Introduced new skb_dst_dev_rcu() function to be used instead of
skb_dst_dev() within rcu_locks in ip_output.This will ensure that
all the skb's associated with the dev being deregistered will
be transnmitted out first, before freeing the dev.

Given that ip_output() is called within an rcu_read_lock()
critical section or from a bottom-half context, it is safe to introduce
an RCU read-side critical section within it.

Multiple panic call stacks were observed when UL traffic was run
in concurrency with device deregistration from different functions,
pasting one sample for reference.

[496733.627565][T13385] Call trace:
[496733.627570][T13385] bpf_prog_ce7c9180c3b128ea_cgroupskb_egres+0x24c/0x7f0
[496733.627581][T13385] __cgroup_bpf_run_filter_skb+0x128/0x498
[496733.627595][T13385] ip_finish_output+0xa4/0xf4
[496733.627605][T13385] ip_output+0x100/0x1a0
[496733.627613][T13385] ip_send_skb+0x68/0x100
[496733.627618][T13385] udp_send_skb+0x1c4/0x384
[496733.627625][T13385] udp_sendmsg+0x7b0/0x898
[496733.627631][T13385] inet_sendmsg+0x5c/0x7c
[496733.627639][T13385] __sys_sendto+0x174/0x1e4
[496733.627647][T13385] __arm64_sys_sendto+0x28/0x3c
[496733.627653][T13385] invoke_syscall+0x58/0x11c
[496733.627662][T13385] el0_svc_common+0x88/0xf4
[496733.627669][T13385] do_el0_svc+0x2c/0xb0
[496733.627676][T13385] el0_svc+0x2c/0xa4
[496733.627683][T13385] el0t_64_sync_handler+0x68/0xb4
[496733.627689][T13385] el0t_64_sync+0x1a4/0x1a8

Changes in v3:
- Replaced WARN_ON() with  WARN_ON_ONCE(), as suggested by Willem de Bruijn.
- Dropped legacy lines mistakenly pulled in from an outdated branch.

Changes in v2:
- Addressed review comments from Eric Dumazet
- Used READ_ONCE() to prevent potential load/store tearing
- Added skb_dst_dev_rcu() and used along with rcu_read_lock() in ip_output

Signed-off-by: Sharath Chandra Vurukala <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Sharath Chandra Vurukala

include/net/dst.h

@@ -568,11 +568,23 @@ static inline struct net_device *dst_dev(const struct dst_entry *dst)
 	return READ_ONCE(dst->dev);
 }
 
+static inline struct net_device *dst_dev_rcu(const struct dst_entry *dst)
+{
+	/* In the future, use rcu_dereference(dst->dev) */
+	WARN_ON_ONCE(!rcu_read_lock_held());
+	return READ_ONCE(dst->dev);
+}
+
 static inline struct net_device *skb_dst_dev(const struct sk_buff *skb)
 {
 	return dst_dev(skb_dst(skb));
 }
 
+static inline struct net_device *skb_dst_dev_rcu(const struct sk_buff *skb)
+{
+	return dst_dev_rcu(skb_dst(skb));
+}
+
 static inline struct net *skb_dst_dev_net(const struct sk_buff *skb)
 {
 	return dev_net(skb_dst_dev(skb));

net/ipv4/ip_output.c

@@ -425,15 +425,20 @@ int ip_mc_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 
 int ip_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
-	struct net_device *dev = skb_dst_dev(skb), *indev = skb->dev;
+	struct net_device *dev, *indev = skb->dev;
+	int ret_val;
 
+	rcu_read_lock();
+	dev = skb_dst_dev_rcu(skb);
 	skb->dev = dev;
 	skb->protocol = htons(ETH_P_IP);
 
-	return NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING,
-			    net, sk, skb, indev, dev,
-			    ip_finish_output,
-			    !(IPCB(skb)->flags & IPSKB_REROUTED));
+	ret_val = NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING,
+				net, sk, skb, indev, dev,
+				ip_finish_output,
+				!(IPCB(skb)->flags & IPSKB_REROUTED));
+	rcu_read_unlock();
+	return ret_val;
 }
 EXPORT_SYMBOL(ip_output);