Subject: Re: [PATCH] net: wwan: replace deprecated strncpy with strscpy_pad

From: Kees Cook <[email-masked]>
Date: Wed, 18 Oct 2023 22:35:26 -0700

On Wed, Oct 18, 2023 at 10:14:55PM +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 chinfo.name to be NUL-terminated based on its use with format
> strings and sprintf:
> rpmsg/rpmsg_char.c
> 165:            dev_err(dev, "failed to open %s\n", eptdev->chinfo.name);
> 368:    return sprintf(buf, "%s\n", eptdev->chinfo.name);
> 
> ... and with strcmp():
> |  static struct rpmsg_endpoint *qcom_glink_create_ept(struct rpmsg_device *rpdev,
> |  						    rpmsg_rx_cb_t cb,
> |  						    void *priv,
> |  						    struct rpmsg_channel_info
> |  									chinfo)
> |  ...
> |  const char *name = chinfo.name;
> |  ...
> |  		if (!strcmp(channel->name, name))
> 
> Moreover, as chinfo is not kzalloc'd, let's opt to NUL-pad the
> destination buffer
> 
> Similar change to:
> Commit 766279a8f85d ("rpmsg: qcom: glink: replace strncpy() with strscpy_pad()")
> and
> Commit 08de420a8014 ("rpmsg: glink: Replace strncpy() with strscpy_pad()")
> 
> Considering the above, a suitable replacement is `strscpy_pad` due to
> the fact that it guarantees both NUL-termination and NUL-padding on the
> destination buffer.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: [email-masked]
> Signed-off-by: Justin Stitt <[email-masked]>
> ---
> Note: build-tested only.
> 
> Found with: $ rg "strncpy\("
> ---
>  drivers/net/wwan/rpmsg_wwan_ctrl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/wwan/rpmsg_wwan_ctrl.c b/drivers/net/wwan/rpmsg_wwan_ctrl.c
> index 86b60aadfa11..39f5e780c478 100644
> --- a/drivers/net/wwan/rpmsg_wwan_ctrl.c
> +++ b/drivers/net/wwan/rpmsg_wwan_ctrl.c
> @@ -37,7 +37,7 @@ static int rpmsg_wwan_ctrl_start(struct wwan_port *port)
>  		.dst = RPMSG_ADDR_ANY,
>  	};

"chinfo" is initialized immediately above here, which means that it is
actually already zero filled for all the members that aren't explicitly
initialized, so the _pad variant isn't needed. I suspect Dead Store
Elimination will optimize it all away anyway, so this is probably fine.

>  
> -	strncpy(chinfo.name, rpwwan->rpdev->id.name, RPMSG_NAME_SIZE);
> +	strscpy_pad(chinfo.name, rpwwan->rpdev->id.name, sizeof(chinfo.name));

Yup, sizeof() replacement looks correct:

struct rpmsg_channel_info {
        char name[RPMSG_NAME_SIZE];

Reviewed-by: Kees Cook <[email-masked]>

-Kees

-- 
Kees Cook