Re: [PATCH] xfs: replace deprecated strncpy with strscpy_pad

[Kees Cook <keescook@chromium.org>]

On Fri, Apr 05, 2024 at 07:52:27PM +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 current code has taken care of NUL-termination by memset()'ing
> @label. This is followed by a strncpy() to perform the string copy.
> 
> Instead, use strscpy_pad() to get both 1) NUL-termination and 2)
> NUL-padding which is needed as this is copied out to userspace.
> 
> Note that this patch uses the new 2-argument version of strscpy_pad
> introduced in Commit e6584c3964f2f ("string: Allow 2-argument
> strscpy()").
> 
> 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>
> ---
> Split from https://lore.kernel.org/all/20240401-strncpy-fs-xfs-xfs_ioctl-c-v1-1-02b9feb1989b@google.com/
> with feedback from Christoph H.
> ---
>  fs/xfs/xfs_ioctl.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index d0e2cec6210d..a1156a8b1e15 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1750,15 +1750,14 @@ xfs_ioc_getlabel(
>  	char			__user *user_label)
>  {
>  	struct xfs_sb		*sbp = &mp->m_sb;
> +	/* 1 larger than sb_fname, for a trailing NUL char */
>  	char			label[XFSLABEL_MAX + 1];
>  
>  	/* Paranoia */
>  	BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);
>  
> -	/* 1 larger than sb_fname, so this ensures a trailing NUL char */
> -	memset(label, 0, sizeof(label));
>  	spin_lock(&mp->m_sb_lock);
> -	strncpy(label, sbp->sb_fname, XFSLABEL_MAX);
> +	strscpy_pad(label, sbp->sb_fname);

Is sbp->sb_fname itself NUL-terminated? This looks like another case of
needing the memtostr() helper?

-Kees

>  	spin_unlock(&mp->m_sb_lock);
>  
>  	if (copy_to_user(user_label, label, sizeof(label)))
> 
> ---
> base-commit: c85af715cac0a951eea97393378e84bb49384734
> change-id: 20240405-strncpy-xfs-split1-a2c408b934c6
> 
> Best regards,
> --
> Justin Stitt <justinstitt@google.com>
> 
> 

-- 
Kees Cook