linux-kernel - Re: [PATCH v3] kobject: Replace strlcpy with strscpy

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Message-ID: <202308301533.D6E65EF5C@keescook>
Date:   Wed, 30 Aug 2023 15:34:52 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Azeem Shaikh <azeemshaikh38@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Laight <David.Laight@...lab.com>,
        linux-hardening@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] kobject: Replace strlcpy with strscpy

On Wed, Aug 30, 2023 at 08:20:21PM +0000, Azeem Shaikh wrote:
> strlcpy() reads the entire source buffer first.
> This read may exceed the destination size limit.
> This is both inefficient and can lead to linear read
> overflows if a source string is not NUL-terminated [1].
> In an effort to remove strlcpy() completely [2], replace
> strlcpy() here with strscpy().
> 
> Direct replacement is safe here since return value of -errno
> is used to check for truncation instead of sizeof(dest).
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> [2] https://github.com/KSPP/linux/issues/89
> 
> Signed-off-by: Azeem Shaikh <azeemshaikh38@...il.com>
> ---
> v3:
>  * Fixed weird indentation.
> 
> v2:
>  * Updated error message to remove @len which is errno now.
>  * https://lore.kernel.org/all/20230830150136.3798414-1-azeemshaikh38@gmail.com/
> 
> v1:
>  * https://lore.kernel.org/all/20230703180528.3709258-1-azeemshaikh38@gmail.com/
> 
>  lib/kobject_uevent.c |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
> index 7c44b7ae4c5c..032d81b959f0 100644
> --- a/lib/kobject_uevent.c
> +++ b/lib/kobject_uevent.c
> @@ -254,10 +254,10 @@ static int init_uevent_argv(struct kobj_uevent_env *env, const char *subsystem)
>  	int buffer_size = sizeof(env->buf) - env->buflen;
>  	int len;
> 
> -	len = strlcpy(&env->buf[env->buflen], subsystem, buffer_size);
> -	if (len >= buffer_size) {
> -		pr_warn("init_uevent_argv: buffer size of %d too small, needed %d\n",
> -			buffer_size, len);
> +	len = strscpy(&env->buf[env->buflen], subsystem, buffer_size);
> +	if (len < 0) {
> +		pr_warn("init_uevent_argv: insufficient buffer space "
> +			"(%u left) for %s\n", buffer_size, subsystem);
>  		return -ENOMEM;
>  	}

I thought scripts/checkpatch.pl would warn about this, but kernel
coding-style (as it is) says to not split strings. I'd expect this to
be:

+		pr_warn("init_uevent_argv: insufficient buffer space (%u left) for %s\n",
+			buffer_size, subsystem);


-- 
Kees Cook