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: d3142c193dca ("sfc: refactor EF10 stats handling")
Reviewed-by: Pieter Jansen van Vuuren <[e-mail]>
Signed-off-by: Edward Cree <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/sfc/ef10.c

···
 {
 	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)
···
 
 	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,