net: vmxnet3: fix possible NULL pointer dereference in vmxnet3_rq_cleanup()

In vmxnet3_rq_create(), when dma_alloc_coherent() fails,
vmxnet3_rq_destroy() is called. It sets rq->rx_ring[i].base to NULL. Then
vmxnet3_rq_create() returns an error to its callers mxnet3_rq_create_all()
-> vmxnet3_change_mtu(). Then vmxnet3_change_mtu() calls
vmxnet3_force_close() -> dev_close() in error handling code. And the driver
calls vmxnet3_close() -> vmxnet3_quiesce_dev() -> vmxnet3_rq_cleanup_all()
-> vmxnet3_rq_cleanup(). In vmxnet3_rq_cleanup(),
rq->rx_ring[ring_idx].base is accessed, but this variable is NULL, causing
a NULL pointer dereference.

To fix this possible bug, an if statement is added to check whether
rq->rx_ring[0].base is NULL in vmxnet3_rq_cleanup() and exit early if so.

The error log in our fault-injection testing is shown as follows:

[   65.220135] BUG: kernel NULL pointer dereference, address: 0000000000000008
...
[   65.222633] RIP: 0010:vmxnet3_rq_cleanup_all+0x396/0x4e0 [vmxnet3]
...
[   65.227977] Call Trace:
...
[   65.228262]  vmxnet3_quiesce_dev+0x80f/0x8a0 [vmxnet3]
[   65.228580]  vmxnet3_close+0x2c4/0x3f0 [vmxnet3]
[   65.228866]  __dev_close_many+0x288/0x350
[   65.229607]  dev_close_many+0xa4/0x480
[   65.231124]  dev_close+0x138/0x230
[   65.231933]  vmxnet3_force_close+0x1f0/0x240 [vmxnet3]
[   65.232248]  vmxnet3_change_mtu+0x75d/0x920 [vmxnet3]
...

Fixes: d1a890fa ("net: VMware virtual Ethernet NIC driver: vmxnet3")
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Zixuan Fu <r33s3n6@gmail.com>
Link: https://lore.kernel.org/r/20220514050711.2636709-1-r33s3n6@gmail.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/vmxnet3/vmxnet3_drv.c

@@ -1668,6 +1668,10 @@ vmxnet3_rq_cleanup(struct vmxnet3_rx_queue *rq,
 	u32 i, ring_idx;
 	struct Vmxnet3_RxDesc *rxd;
 
+	/* ring has already been cleaned up */
+	if (!rq->rx_ring[0].base)
+		return;
+
 	for (ring_idx = 0; ring_idx < 2; ring_idx++) {
 		for (i = 0; i < rq->rx_ring[ring_idx].size; i++) {
 #ifdef __BIG_ENDIAN_BITFIELD