[PATCH 2/5] idpf: skip changing MTU if vport is NULL during HW reset

[Li Li <boolli@google.com>]

When an idpf HW reset is triggered, it clears the vport but does
not clear the netdev held by vport:

    // In idpf_vport_dealloc() called by idpf_init_hard_reset(),
    // idpf_init_hard_reset() sets IDPF_HR_RESET_IN_PROG, so
    // idpf_decfg_netdev() doesn't get called.
    if (!test_bit(IDPF_HR_RESET_IN_PROG, adapter->flags))
        idpf_decfg_netdev(vport);
    // idpf_decfg_netdev() would clear netdev but it isn't called:
    unregister_netdev(vport->netdev);
    free_netdev(vport->netdev);
    vport->netdev = NULL;
    // Later in idpf_init_hard_reset(), the vport is cleared:
    kfree(adapter->vports);
    adapter->vports = NULL;

During an idpf HW reset, when userspace changes the MTU of the netdev,
the vport associated with the netdev is NULL, and so a kernel panic
would happen:

[ 2081.955742] BUG: kernel NULL pointer dereference, address: 0000000000000068
...
[ 2082.002739] RIP: 0010:idpf_initiate_soft_reset+0x19/0x190

This can be reproduced reliably by injecting a TX timeout to cause
an idpf HW reset, and injecting a virtchnl error to cause the HW
reset to fail and retry, while changing the MTU of the netdev in
userspace.

With this patch applied, we see the following error but no kernel
panics anymore:

[  304.291346] idpf 0000:05:00.0 eth1: mtu not changed due to no vport innetdev
RTNETLINK answers: Bad address

Signed-off-by: Li Li <boolli@google.com>
---
 drivers/net/ethernet/intel/idpf/idpf_lib.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/ethernet/intel/idpf/idpf_lib.c b/drivers/net/ethernet/intel/idpf/idpf_lib.c
index 57b8b3fd9124c..53b31989722a7 100644
--- a/drivers/net/ethernet/intel/idpf/idpf_lib.c
+++ b/drivers/net/ethernet/intel/idpf/idpf_lib.c
@@ -2328,11 +2327,17 @@ static int idpf_change_mtu(struct net_device *netdev, int new_mtu)
 
 	idpf_vport_ctrl_lock(netdev);
 	vport = idpf_netdev_to_vport(netdev);
+	if (!vport) {
+		netdev_err(netdev, "mtu not changed due to no vport in netdev\n");
+		err = -EFAULT;
+		goto unlock;
+	}
 
 	WRITE_ONCE(netdev->mtu, new_mtu);
 
 	err = idpf_initiate_soft_reset(vport, IDPF_SR_MTU_CHANGE);
 
+unlock:
 	idpf_vport_ctrl_unlock(netdev);
 
 	return err;
-- 
2.52.0.351.gbe84eed79e-goog