[PATCH 5.10] vxlan: Fix potential null-ptr-deref in vxlan_gro_prepare_receive().

[Artem Novikov <naa@amicon.ru>]

From: Kuniyuki Iwashima <kuniyu@google.com>

commit 30a45c0bffdd62350261e2f2689fdba426a33578 upstream.

udp_tunnel_sock_release() could set sk->sk_user_data to NULL
while vxlan_gro_prepare_receive() is running.

Let's check if rcu_dereference_sk_user_data() is NULL after
skb_gro_remcsum_init().

Fixes: 5602c48cf875 ("vxlan: change vxlan to use UDP socket GRO")
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260502031401.3557229-7-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Artem Novikov: Apply the check to the older vxlan_gro_receive()
  implementation. Initialize the remcsum state first and use the
  existing out path for cleanup when sk_user_data has been cleared. ]
Signed-off-by: Artem Novikov <naa@amicon.ru>
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
---
 drivers/net/vxlan/vxlan_core.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index 8dad17023e..7a916eebf5 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -790,12 +790,16 @@ static struct sk_buff *vxlan_gro_receive(struct sock *sk,
 	struct vxlanhdr *vh, *vh2;
 	unsigned int hlen, off_vx;
 	int flush = 1;
-	struct vxlan_sock *vs = rcu_dereference_sk_user_data(sk);
+	struct vxlan_sock *vs;
 	__be32 flags;
 	struct gro_remcsum grc;
 
 	skb_gro_remcsum_init(&grc);
 
+	vs = rcu_dereference_sk_user_data(sk);
+	if (!vs)
+		goto out;
+
 	off_vx = skb_gro_offset(skb);
 	hlen = off_vx + sizeof(*vh);
 	vh   = skb_gro_header_fast(skb, off_vx);