[PATCH] net: cpmac: replace deprecated strncpy with strscpy

[Justin Stitt <justinstitt@google.com>]

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

We expect mdio_bus_id to be NUL-terminated based on its use with
snprint():
|       snprintf(priv->phy_name, MII_BUS_ID_SIZE, PHY_ID_FMT,
|                                               mdio_bus_id, phy_id);

Moreover, as this is the only use of mdio_bus_id, we can see that
NUL-padding is not required.

Considering the above, a suitable replacement is `strscpy` [2] due to
the fact that it guarantees NUL-termination on the destination buffer
without unnecessarily NUL-padding.

Note that for the replacement involving cpmac_mii->id, the source
buffer's length is equal to MII_BUS_ID_SIZE which could result in a
buffer overread. However, there is no buffer overread since "cpmac-1"
(the string copied into cpmac_mii->id) is smaller than MII_BUS_ID_SIZE
thus meaning the previous usage of strncpy() here did _not_ have any
overread bugs. Nonetheless, let's still favor strscpy() over strncpy().

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Note: build-tested only.

Found with: $ rg "strncpy\("
---
 drivers/net/ethernet/ti/cpmac.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/ti/cpmac.c b/drivers/net/ethernet/ti/cpmac.c
index 80eeeb463c4f..714328ee7487 100644
--- a/drivers/net/ethernet/ti/cpmac.c
+++ b/drivers/net/ethernet/ti/cpmac.c
@@ -1068,7 +1068,7 @@ static int cpmac_probe(struct platform_device *pdev)
 	pdata = dev_get_platdata(&pdev->dev);
 
 	if (external_switch || dumb_switch) {
-		strncpy(mdio_bus_id, "fixed-0", MII_BUS_ID_SIZE); /* fixed phys bus */
+		strscpy(mdio_bus_id, "fixed-0", sizeof(mdio_bus_id)); /* fixed phys bus */
 		phy_id = pdev->id;
 	} else {
 		for (phy_id = 0; phy_id < PHY_MAX_ADDR; phy_id++) {
@@ -1076,7 +1076,8 @@ static int cpmac_probe(struct platform_device *pdev)
 				continue;
 			if (!mdiobus_get_phy(cpmac_mii, phy_id))
 				continue;
-			strncpy(mdio_bus_id, cpmac_mii->id, MII_BUS_ID_SIZE);
+			strscpy(mdio_bus_id, cpmac_mii->id,
+				sizeof(mdio_bus_id));
 			break;
 		}
 	}
@@ -1084,10 +1085,9 @@ static int cpmac_probe(struct platform_device *pdev)
 	if (phy_id == PHY_MAX_ADDR) {
 		dev_err(&pdev->dev, "no PHY present, falling back "
 			"to switch on MDIO bus 0\n");
-		strncpy(mdio_bus_id, "fixed-0", MII_BUS_ID_SIZE); /* fixed phys bus */
+		strscpy(mdio_bus_id, "fixed-0", sizeof(mdio_bus_id)); /* fixed phys bus */
 		phy_id = pdev->id;
 	}
-	mdio_bus_id[sizeof(mdio_bus_id) - 1] = '\0';
 
 	dev = alloc_etherdev_mq(sizeof(*priv), CPMAC_QUEUES);
 	if (!dev)

---
base-commit: cbf3a2cb156a2c911d8f38d8247814b4c07f49a2
change-id: 20231012-strncpy-drivers-net-ethernet-ti-cpmac-c-9d9d1b33d595

Best regards,
--
Justin Stitt <justinstitt@google.com>