igb: Do not free q_vector unless new one was allocated

Avoid potential use-after-free condition under memory pressure. If the
kzalloc() fails, q_vector will be freed but left in the original
adapter->q_vector[v_idx] array position.

Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc: Tony Nguyen <anthony.l.nguyen@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: intel-wired-lan@lists.osuosl.org
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -1202,8 +1202,12 @@ static int igb_alloc_q_vector(struct igb_adapter *adapter,
 	if (!q_vector) {
 		q_vector = kzalloc(size, GFP_KERNEL);
 	} else if (size > ksize(q_vector)) {
-		kfree_rcu(q_vector, rcu);
-		q_vector = kzalloc(size, GFP_KERNEL);
+		struct igb_q_vector *new_q_vector;
+
+		new_q_vector = kzalloc(size, GFP_KERNEL);
+		if (new_q_vector)
+			kfree_rcu(q_vector, rcu);
+		q_vector = new_q_vector;
 	} else {
 		memset(q_vector, 0, size);
 	}