[bug report] net: mediatek: mtk_eth_sock: allocate dummy net_device dynamically

[bug report] net: mediatek: mtk_eth_sock: allocate dummy net_device dynamically

[Dan Carpenter]

Hello Breno Leitao,

Commit b209bd6d0bff ("net: mediatek: mtk_eth_sock: allocate dummy
net_device dynamically") from Apr 22, 2024 (linux-next), leads to the
following Smatch static checker warning:

	drivers/net/ethernet/mediatek/mtk_eth_soc.c:5061 mtk_probe()
	error: we previously assumed 'eth->dummy_dev' could be null (see line 5041)

drivers/net/ethernet/mediatek/mtk_eth_soc.c
    5037         /* we run 2 devices on the same DMA ring so we need a dummy device
    5038          * for NAPI to work
    5039          */
    5040         eth->dummy_dev = alloc_netdev_dummy(0);
    5041         if (!eth->dummy_dev) {
                      ^^^^^^^^^^^^^^
->dummy_dev is the last allocation

    5042                 err = -ENOMEM;
    5043                 dev_err(eth->dev, "failed to allocated dummy device\n");
    5044                 goto err_unreg_netdev;
    5045         }
    5046         netif_napi_add(eth->dummy_dev, &eth->tx_napi, mtk_napi_tx);
    5047         netif_napi_add(eth->dummy_dev, &eth->rx_napi, mtk_napi_rx);
    5048 
    5049         platform_set_drvdata(pdev, eth);
    5050         schedule_delayed_work(&eth->reset.monitor_work,
    5051                               MTK_DMA_MONITOR_TIMEOUT);
    5052 
    5053         return 0;
    5054 
    5055 err_unreg_netdev:
    5056         mtk_unreg_dev(eth);
    5057 err_deinit_ppe:
    5058         mtk_ppe_deinit(eth);
    5059         mtk_mdio_cleanup(eth);
    5060 err_free_dev:
--> 5061         mtk_free_dev(eth);
                              ^^^
But it's free here.  (NULL dereference).  I really would suggest moving the
free_netdev() to the release function.  The probe and release function are
easier to read if they're in mirrored order where every allocation has a
matching free.

    5062 err_deinit_hw:
    5063         mtk_hw_deinit(eth);
    5064 err_wed_exit:
    5065         mtk_wed_exit();
    5066 err_destroy_sgmii:
    5067         mtk_sgmii_destroy(eth);
    5068 
    5069         return err;
    5070 }

regards,
dan carpenter

Re: [bug report] net: mediatek: mtk_eth_sock: allocate dummy net_device dynamically

[Breno Leitao]

Hello Dan,

On Fri, Jul 19, 2024 at 06:52:50PM -0500, Dan Carpenter wrote:
> But it's free here.  (NULL dereference).  I really would suggest moving the
> free_netdev() to the release function.  The probe and release function are
> easier to read if they're in mirrored order where every allocation has a
> matching free.

Thanks for reporting it. It seems a real problem at the error path,
indeed.

I've hacked a patch with your suggestion. how does it look like?


Author: Breno Leitao <leitao@debian.org>
Date:   Mon Jul 22 02:14:19 2024 -0700

    net: mediatek: Fix potential NULL pointer dereference in dummy net_device handling
    
    Move the freeing of the dummy net_device from mtk_free_dev() to mtk_remove().
    
    Previously, if alloc_netdev_dummy() failed in mtk_probe(), eth->dummy_dev
    would be NULL. The error path would then call mtk_free_dev(), which in turn
    called free_netdev() assuming dummy_dev was allocated (but it was not),
    potentially causing a NULL pointer dereference.
    
    By moving free_netdev() to mtk_remove(), we ensure it's only called when
    mtk_probe() has succeeded and dummy_dev is fully allocated. This addresses
    a potential NULL pointer dereference detected by Smatch[1].
    
    Link: https://lore.kernel.org/all/4160f4e0-cbef-4a22-8b5d-42c4d399e1f7@stanley.mountain/ [1]
    Fixes: b209bd6d0bff ("net: mediatek: mtk_eth_sock: allocate dummy net_device dynamically")
    Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
    Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
    Signed-off-by: Breno Leitao <leitao@debian.org>

diff --git a/drivers/net/ethernet/mediatek/mtk_eth_soc.c b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
index 0cc2dd85652f..16ca427cf4c3 100644
--- a/drivers/net/ethernet/mediatek/mtk_eth_soc.c
+++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
@@ -4223,8 +4223,6 @@ static int mtk_free_dev(struct mtk_eth *eth)
 		metadata_dst_free(eth->dsa_meta[i]);
 	}
 
-	free_netdev(eth->dummy_dev);
-
 	return 0;
 }
 
@@ -5090,6 +5088,7 @@ static void mtk_remove(struct platform_device *pdev)
 	netif_napi_del(&eth->tx_napi);
 	netif_napi_del(&eth->rx_napi);
 	mtk_cleanup(eth);
+	free_netdev(eth->dummy_dev);
 	mtk_mdio_cleanup(eth);
 }
 

Re: [bug report] net: mediatek: mtk_eth_sock: allocate dummy net_device dynamically

[Dan Carpenter]

On Mon, Jul 22, 2024 at 02:28:08AM -0700, Breno Leitao wrote:
> Hello Dan,
> 
> On Fri, Jul 19, 2024 at 06:52:50PM -0500, Dan Carpenter wrote:
> > But it's free here.  (NULL dereference).  I really would suggest moving the
> > free_netdev() to the release function.  The probe and release function are
> > easier to read if they're in mirrored order where every allocation has a
> > matching free.
> 
> Thanks for reporting it. It seems a real problem at the error path,
> indeed.
> 
> I've hacked a patch with your suggestion. how does it look like?
> 

Looks great, thanks!

Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>

regards,
dan carpenter