vxlan_gro_prepare_receive() reads vs->flags from the vxlan_sock returned
by rcu_dereference_sk_user_data(sk) without a NULL check, unlike the rest
of the driver (e.g. vxlan_rcv() does "if (!vs) goto drop;").

udp_tunnel_sock_release() clears sk_user_data to NULL and waits an RCU
grace period before the socket leaves the hash, so any callback running on
the tunnel socket during teardown must tolerate a NULL sk_user_data. If a
VXLAN device is torn down while GRO runs on its UDP tunnel socket,
sk_user_data can be NULL when a packet carrying VXLAN_HF_RCO arrives,
faulting in NAPI/softirq context.

The GRO path was the only sk_user_data reader on the socket missing this
guard (vxlan_rcv and vxlan_err_lookup already have it; the gro_complete
callbacks do not touch sk_user_data). Add the same NULL check so GRO
flushes instead of dereferencing a NULL socket pointer.

  Oops: general protection fault ... SMP KASAN NOPTI
  KASAN: probably user-memory-access in range [0x2018-0x201f]
  RIP: 0010:vxlan_gro_prepare_receive (drivers/net/vxlan/vxlan_core.c:678)
   vxlan_gro_receive (drivers/net/vxlan/vxlan_core.c:713)
   udp_gro_receive (net/ipv4/udp_offload.c:841)
   ...
   dev_gro_receive (net/core/gro.c:522)
   gro_receive_skb (net/core/gro.c:640)
   tun_napi_poll (drivers/net/tun.c:259)
   ...
  Kernel panic - not syncing: Fatal exception in interrupt

Fixes: 5602c48cf875 ("vxlan: change vxlan to use UDP socket GRO")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Xiang Mei (Microsoft) <redacted>
---
 drivers/net/vxlan/vxlan_core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index b5b1253ac08b..f2235bae9bfe 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c

@@ -663,6 +663,9 @@ static struct vxlanhdr *vxlan_gro_prepare_receive(struct sock *sk,
 	struct vxlan_sock *vs = rcu_dereference_sk_user_data(sk);
 	__be32 flags;
 
+	if (!vs)
+		return NULL;
+
 	skb_gro_remcsum_init(grc);
 
 	off_vx = skb_gro_offset(skb);

-- 
2.43.0