[PATCH v2] net: mdio_bus: replace deprecated strncpy with strscpy

[PATCH v2] net: mdio_bus: replace deprecated strncpy with strscpy

[Justin Stitt]

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 mdiodev->modalias to be NUL-terminated based on its usage with
strcmp():
|       return strcmp(mdiodev->modalias, drv->name) == 0;

Moreover, mdiodev->modalias is already zero-allocated:
|       mdiodev = kzalloc(sizeof(*mdiodev), GFP_KERNEL);
... which means the NUL-padding strncpy provides is not necessary.

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.

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>
---
Changes in v2:
- rename subject line as it was the same as another and was causing
  problems.
- rebased onto mainline bee0e7762ad2c602
- Link to v1 (lore): https://lore.kernel.org/r/20231012-strncpy-drivers-net-phy-mdio_bus-c-v1-1-15242e6f9ec4@google.com
- Link to v1 (patchwork): https://patchwork.kernel.org/project/netdevbpf/patch/20231012-strncpy-drivers-net-phy-mdio_bus-c-v1-1-15242e6f9ec4@google.com/
- Link to collided: https://patchwork.kernel.org/project/netdevbpf/patch/20231012-strncpy-drivers-net-mdio-mdio-gpio-c-v1-1-ab9b06cfcdab@google.com/
---
Note: build-tested only.

Found with: $ rg "strncpy\("
---
 drivers/net/phy/mdio_bus.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/phy/mdio_bus.c b/drivers/net/phy/mdio_bus.c
index 25dcaa49ab8b..6cf73c15635b 100644
--- a/drivers/net/phy/mdio_bus.c
+++ b/drivers/net/phy/mdio_bus.c
@@ -506,7 +506,7 @@ static int mdiobus_create_device(struct mii_bus *bus,
 	if (IS_ERR(mdiodev))
 		return -ENODEV;
 
-	strncpy(mdiodev->modalias, bi->modalias,
+	strscpy(mdiodev->modalias, bi->modalias,
 		sizeof(mdiodev->modalias));
 	mdiodev->bus_match = mdio_device_bus_match;
 	mdiodev->dev.platform_data = (void *)bi->platform_data;

---
base-commit: bee0e7762ad2c6025b9f5245c040fcc36ef2bde8
change-id: 20231012-strncpy-drivers-net-phy-mdio_bus-c-0a0d5e875712

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

Re: [PATCH v2] net: mdio_bus: replace deprecated strncpy with strscpy

[Kees Cook]

On Thu, Dec 07, 2023 at 09:57:50PM +0000, Justin Stitt wrote:
> 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 mdiodev->modalias to be NUL-terminated based on its usage with
> strcmp():
> |       return strcmp(mdiodev->modalias, drv->name) == 0;
> 
> Moreover, mdiodev->modalias is already zero-allocated:
> |       mdiodev = kzalloc(sizeof(*mdiodev), GFP_KERNEL);
> ... which means the NUL-padding strncpy provides is not necessary.
> 
> 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.
> 
> 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>

Yeah, the subject is distinct now. :)

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook

Re: [PATCH v2] net: mdio_bus: replace deprecated strncpy with strscpy

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by David S. Miller <davem@davemloft.net>:

On Thu, 07 Dec 2023 21:57:50 +0000 you wrote:
> 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 mdiodev->modalias to be NUL-terminated based on its usage with
> strcmp():
> |       return strcmp(mdiodev->modalias, drv->name) == 0;
> 
> [...]

Here is the summary with links:
  - [v2] net: mdio_bus: replace deprecated strncpy with strscpy
    https://git.kernel.org/netdev/net-next/c/1674110c0dd4

You are awesome, thank you!
-- 
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