[PATCH net-next] ip_tunnel: Fix DST_METADATA dst_entry handle in tnl_update_pmtu

[wenxu@ucloud.cn]

From: wenxu <wenxu@ucloud.cn>

BUG report in selftests: bpf: test_tunnel.sh

Testing IPIP tunnel...
BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
PGD 0 P4D 0
Oops: 0010 [#1] SMP PTI
CPU: 0 PID: 16822 Comm: ping Not tainted 5.0.0-rc3-00352-gc8b34e6 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
RIP: 0010:          (null)
Code: Bad RIP value.
RSP: 0018:ffffc9000104f9c8 EFLAGS: 00010286
RAX: 0000000000000000 RBX: ffffe8ffffc071a8 RCX: 0000000000000000
RDX: ffff888054e33000 RSI: ffff88807796f500 RDI: ffffe8ffffc07130
RBP: ffff88807796f500 R08: ffff88806da4f0a0 R09: 0000000000000000
R10: 0000000000000004 R11: ffff888054e33000 R12: 0000000000000054
R13: ffff88805e714000 R14: ffff88806da4f0a0 R15: 0000000000000000
FS:  00007f4c00431500(0000) GS:ffff88813fc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000008276e000 CR4: 00000000000406f0
Call Trace:
 ? tnl_update_pmtu+0x21b/0x250 [ip_tunnel]
 ? ip_md_tunnel_xmit+0x1b7/0xdc0 [ip_tunnel]
 ? ipip_tunnel_xmit+0x90/0xc0 [ipip]
 ? dev_hard_start_xmit+0x98/0x210
 ? __dev_queue_xmit+0x6a9/0x8e0

The bpf program set tunnel_key through bpf_skb_set_tunnel_key which will
drop the old dst_entry and create a DST_METADATA dst_entry. It will lead
the tunnel_update_pmtu operator the dst_entry incorrect. So It should be
check the dst_entry is valid.

Fixes: c8b34e680a09 ("ip_tunnel: Add tnl_update_pmtu in ip_md_tunnel_xmit")
Signed-off-by: wenxu <wenxu@ucloud.cn>
---
 net/ipv4/ip_tunnel.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index 893f013..a665f11 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -515,7 +515,7 @@ static int tnl_update_pmtu(struct net_device *dev, struct sk_buff *skb,
 		mtu = dst_mtu(&rt->dst) - dev->hard_header_len
 					- sizeof(struct iphdr) - tunnel_hlen;
 	else
-		mtu = skb_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
+		mtu = skb_valid_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
 
 	skb_dst_update_pmtu(skb, mtu);
 
@@ -530,7 +530,7 @@ static int tnl_update_pmtu(struct net_device *dev, struct sk_buff *skb,
 	}
 #if IS_ENABLED(CONFIG_IPV6)
 	else if (skb->protocol == htons(ETH_P_IPV6)) {
-		struct rt6_info *rt6 = (struct rt6_info *)skb_dst(skb);
+		struct rt6_info *rt6 = (struct rt6_info *)skb_valid_dst(skb);
 		__be32 daddr;
 
 		daddr = md ? dst : tunnel->parms.iph.daddr;
-- 
1.8.3.1