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 outout.This will ensure that
all the skb's associated with the dev being deregistered will
be transnmitted out first, before freeing the dev.

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 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 <quic_sharathv@quicinc.com>
---
 include/net/dst.h    | 12 ++++++++++++
 net/ipv4/ip_output.c | 17 ++++++++++++-----
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/include/net/dst.h b/include/net/dst.h
index 00467c1b5093..692ebb1b3f42 100644
--- a/include/net/dst.h
+++ b/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(!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));
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 10a1d182fd84..d70d79b71897 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -425,15 +425,22 @@ 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;
 
+	IP_UPD_PO_STATS(net, IPSTATS_MIB_OUT, skb->len);
+
+	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);