linux-hardening - Re: [PATCH] Squashfs: replace deprecated strncpy with strscpy

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Date: Thu, 28 Mar 2024 20:48:57 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Phillip Lougher <phillip@...ashfs.org.uk>, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] Squashfs: replace deprecated strncpy with strscpy

On Thu, Mar 28, 2024 at 09:52:59PM +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.
> 
> The previous code took special care of NUL-terminating the destination
> buffer via a manual assignment. As such, there is no bug in the current
> implementation. However, in an effort to rid the kernel of strscpy()

typo: rid kernel of strncpy. :)

> [2], Let's instead use strscpy() which guarantees this behavior [3]. To
> ensure we can copy the same number of bytes as before, add 1 to the
> length argument provided to strscpy().
> 
> 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 [2]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html  [3]
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Note: build-tested only.
> 
> Found with: $ rg "strncpy\("
> ---
>  fs/squashfs/namei.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/fs/squashfs/namei.c b/fs/squashfs/namei.c
> index 11e4539b9eae..6c4704ba8f42 100644
> --- a/fs/squashfs/namei.c
> +++ b/fs/squashfs/namei.c
> @@ -80,8 +80,7 @@ static int get_dir_index_using_name(struct super_block *sb,
>  	}
>  
>  	str = &index->name[SQUASHFS_NAME_LEN + 1];
> -	strncpy(str, name, len);
> -	str[len] = '\0';
> +	strscpy(str, name, len + 1);

Otherwise, yeah, looks right.

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

-Kees

>  
>  	for (i = 0; i < i_count; i++) {
>  		err = squashfs_read_metadata(sb, index, &index_start,
> 
> ---
> base-commit: 928a87efa42302a23bb9554be081a28058495f22
> change-id: 20240328-strncpy-fs-squashfs-namei-c-9d01b8975e53
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 
> 

-- 
Kees Cook