[PATCH net v3 3/5] gve: Block invalid ring sizes when device options are 0

[Pin-yen Lin <treapking@google.com>]

On older device versions, minimum ring sizes in the device option are
reported as 0. The driver imports these 0 values, allowing userspace
(`ethtool -G`) to request invalidly small ring sizes.

This issue was discovered and actually triggered by the
drivers/net/ring_reconfig.py selftest (ringparam test case). Halving the
ring size down to 1 triggered a kernel NULL pointer dereference in
gve_tx_alloc_rings_dqo():

  BUG: kernel NULL pointer dereference, address: 0000000000000002
  RIP: 0010:gve_tx_alloc_rings_dqo+0x1c4/0x510 [gve]
  Call Trace:
   <TASK>
   gve_queues_mem_alloc+0x29/0xa0 [gve]
   gve_adjust_config+0x1b/0x140 [gve]
   gve_set_ringparam+0x1f4/0x250 [gve]
   ethnl_set_rings+0x260/0x440
   ...

Fix this by falling back to the driver's default minimum ring sizes
(GVE_DEFAULT_MIN_RX_RING_SIZE and GVE_DEFAULT_MIN_TX_RING_SIZE) whenever
the device option reports minimum ring sizes as 0.

This change was tested using the DQO RDA queue format.

Cc: stable@vger.kernel.org
Fixes: ed4fb326947d ("gve: add support to read ring size ranges from the device")
Signed-off-by: Pin-yen Lin <treapking@google.com>
Reviewed-by: Joshua Washington <joshwash@google.com>
Reviewed-by: Jordan Rhee <jordanrhee@google.com>
---

Changes in v3:
- Retitled and refined the commit message

 drivers/net/ethernet/google/gve/gve_adminq.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/google/gve/gve_adminq.c b/drivers/net/ethernet/google/gve/gve_adminq.c
index f05f4895f4c7..5e2f91b63335 100644
--- a/drivers/net/ethernet/google/gve/gve_adminq.c
+++ b/drivers/net/ethernet/google/gve/gve_adminq.c
@@ -189,7 +189,9 @@ void gve_parse_device_option(struct gve_priv *priv,
 		*dev_op_modify_ring = (void *)(option + 1);
 
 		/* device has not provided min ring size */
-		if (option_length == GVE_DEVICE_OPTION_NO_MIN_RING_SIZE)
+		if (option_length == GVE_DEVICE_OPTION_NO_MIN_RING_SIZE ||
+		    be16_to_cpu((*dev_op_modify_ring)->min_rx_ring_size) == 0 ||
+		    be16_to_cpu((*dev_op_modify_ring)->min_tx_ring_size) == 0)
 			priv->default_min_ring_size = true;
 		break;
 	case GVE_DEV_OPT_ID_FLOW_STEERING:
-- 
2.56.0.385.gd3acb90ef8-goog