sfc: fix crash when reading stats while NIC is resetting

efx_net_stats() (.ndo_get_stats64) can be called during an ethtool
 selftest, during which time nic_data->mc_stats is NULL as the NIC has
 been fini'd.  In this case do not attempt to fetch the latest stats
 from the hardware, else we will crash on a NULL dereference:
    BUG: kernel NULL pointer dereference, address: 0000000000000038
    RIP efx_nic_update_stats
    abridged calltrace:
    efx_ef10_update_stats_pf
    efx_net_stats
    dev_get_stats
    dev_seq_printf_stats
Skipping the read is safe, we will simply give out stale stats.
To ensure that the free in efx_ef10_fini_nic() does not race against
 efx_ef10_update_stats_pf(), which could cause a TOCTTOU bug, take the
 efx->stats_lock in fini_nic (it is already held across update_stats).

Fixes: d3142c19 ("sfc: refactor EF10 stats handling")
Reviewed-by: Pieter Jansen van Vuuren <pieter.jansen-van-vuuren@amd.com>
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/ef10.c

@@ -1297,8 +1297,10 @@ static void efx_ef10_fini_nic(struct efx_nic *efx)
 {
 	struct efx_ef10_nic_data *nic_data = efx->nic_data;
 
+	spin_lock_bh(&efx->stats_lock);
 	kfree(nic_data->mc_stats);
 	nic_data->mc_stats = NULL;
+	spin_unlock_bh(&efx->stats_lock);
 }
 
 static int efx_ef10_init_nic(struct efx_nic *efx)
@@ -1852,9 +1854,14 @@ static size_t efx_ef10_update_stats_pf(struct efx_nic *efx, u64 *full_stats,
 
 	efx_ef10_get_stat_mask(efx, mask);
 
-	efx_nic_copy_stats(efx, nic_data->mc_stats);
-	efx_nic_update_stats(efx_ef10_stat_desc, EF10_STAT_COUNT,
-			     mask, stats, nic_data->mc_stats, false);
+	/* If NIC was fini'd (probably resetting), then we can't read
+	 * updated stats right now.
+	 */
+	if (nic_data->mc_stats) {
+		efx_nic_copy_stats(efx, nic_data->mc_stats);
+		efx_nic_update_stats(efx_ef10_stat_desc, EF10_STAT_COUNT,
+				     mask, stats, nic_data->mc_stats, false);
+	}
 
 	/* Update derived statistics */
 	efx_nic_fix_nodesc_drop_stat(efx,