linux-hardening - Re: [PATCH] s390/ctcm: replace deprecated strncpy with strscpy

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Date: Tue, 24 Oct 2023 18:32:45 +0200
From: Thorsten Winkler <twinkler@...ux.ibm.com>
To: Justin Stitt <justinstitt@...gle.com>,
        Alexandra Winter <wintera@...ux.ibm.com>,
        Wenjia Zhang
 <wenjia@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev
 <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>
Cc: linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] s390/ctcm: replace deprecated strncpy with strscpy



On 23.10.23 21:35, 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 chid to be NUL-terminated based on its use with format
> strings:
> 
> 	CTCM_DBF_TEXT_(SETUP, CTC_DBF_INFO, "%s(%s) %s", CTCM_FUNTAIL,
> 			chid, ok ? "OK" : "failed");
> 
> Moreover, NUL-padding is not required as it is _only_ used in this one
> instance with a format string.
> 
> 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.
> 
> We can also drop the +1 from chid's declaration as we no longer need to
> be cautious about leaving a spot for a NUL-byte. Let's use the more
> idiomatic strscpy usage of (dest, src, sizeof(dest)) as this more
> closely ties the destination buffer to the length.
> 
> 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@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>

LGTM, thank you!

Reviewed-by: Thorsten Winkler <twinkler@...ux.ibm.com>
Tested-by: Thorsten Winkler <twinkler@...ux.ibm.com>

> ---
> Note: build-tested only.
> 
> Found with: $ rg "strncpy\("
> ---
>   drivers/s390/net/ctcm_main.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/s390/net/ctcm_main.c b/drivers/s390/net/ctcm_main.c
> index 6faf27136024..ac15d7c2b200 100644
> --- a/drivers/s390/net/ctcm_main.c
> +++ b/drivers/s390/net/ctcm_main.c
> @@ -200,13 +200,13 @@ static void channel_free(struct channel *ch)
>   static void channel_remove(struct channel *ch)
>   {
>   	struct channel **c = &channels;
> -	char chid[CTCM_ID_SIZE+1];
> +	char chid[CTCM_ID_SIZE];
>   	int ok = 0;
>   
>   	if (ch == NULL)
>   		return;
>   	else
> -		strncpy(chid, ch->id, CTCM_ID_SIZE);
> +		strscpy(chid, ch->id, sizeof(chid));
>   
>   	channel_free(ch);
>   	while (*c) {
> 
> ---
> base-commit: 9c5d00cb7b6bbc5a7965d9ab7d223b5402d1f02c
> change-id: 20231023-strncpy-drivers-s390-net-ctcm_main-c-f9180f470c69
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 
>