[PATCH net v2] netdevsim: fix panic when NETIF_F_LOOPBACK is set on a VF port

[Jiayuan Chen <jiayuan.chen@linux.dev>]

A netdevsim VF port cannot process incoming data.  It never runs
nsim_queue_init(), so ns->rq is NULL, and nsim_vf_netdev_ops has no
->ndo_open, so its NAPI is never set up either.

nsim_setup() offers NETIF_F_LOOPBACK to both PF and VF ports.  Turning
it on for a VF makes nsim_start_xmit() take ns itself as peer_ns and
dereference peer_ns->rq[], which panics.

A VF cannot deliver packets at all: it can't be linked as a peer either,
since netdev_is_nsim() only matches the PF netdev_ops, so ns->peer is
always NULL and every skb ends up dropped.  Give VF ports a dedicated
xmit that just drops the traffic, so VFs no longer depend on PF-only
state in nsim_start_xmit().

Reproduce in qemu:
  modprobe netdevsim
  echo "90 1" > /sys/bus/netdevsim/new_device
  echo 2 > /sys/bus/netdevsim/devices/netdevsim90/sriov_numvfs
  devlink dev eswitch set netdevsim/netdevsim90 mode switchdev
  ethtool -K eth2 loopback on   # eth2 is the vfnum 0 port
  ip link set eth2 up

Panic:
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  CPU: 0 UID: 0 PID: 141 Comm: kworker/0:3 Not tainted 7.2.0+ #338 PREEMPT
  Workqueue: mld mld_ifc_work
  RIP: 0010:nsim_start_xmit (drivers/net/netdevsim/netdev.c:159)
  dev_hard_start_xmit ( net/core/dev.c:3953)
  sch_direct_xmit (net/sched/sch_generic.c:372)
  __dev_queue_xmit (net/core/dev.c:4262 net/core/dev.c:4884)
  neigh_resolve_output (net/core/neighbour.c:1616)
  ip6_finish_output2 (net/ipv6/ip6_output.c:138)
  ip6_finish_output (net/ipv6/ip6_output.c:221)
  ip6_output (net/ipv6/ip6_output.c:248)
  ......

Fixes: 358008f41d9b ("netdevsim: add loopback support")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>

---
v2: use a dedicated drop-only xmit for VF ports instead of clearing
    NETIF_F_LOOPBACK, per Jakub
v1: https://lore.kernel.org/netdev/20260831042221.137137-1-jiayuan.chen@linux.dev/
---
 drivers/net/netdevsim/netdev.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index b4a99f3ceac6..eebc02ccc4a9 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -185,6 +185,14 @@ static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	return NETDEV_TX_OK;
 }
 
+static netdev_tx_t nsim_start_xmit_vf(struct sk_buff *skb,
+				      struct net_device *dev)
+{
+	dev_dstats_tx_dropped(dev);
+	kfree_skb(skb);
+	return NETDEV_TX_OK;
+}
+
 static int nsim_set_rx_mode(struct net_device *dev,
 			    struct netdev_hw_addr_list *uc,
 			    struct netdev_hw_addr_list *mc)
@@ -651,7 +659,7 @@ static const struct net_device_ops nsim_netdev_ops = {
 };
 
 static const struct net_device_ops nsim_vf_netdev_ops = {
-	.ndo_start_xmit		= nsim_start_xmit,
+	.ndo_start_xmit		= nsim_start_xmit_vf,
 	.ndo_set_rx_mode_async	= nsim_set_rx_mode,
 	.ndo_set_mac_address	= eth_mac_addr,
 	.ndo_validate_addr	= eth_validate_addr,
-- 
2.43.0