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

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Date: Wed, 3 Apr 2024 20:31:59 +0100
From: Phillip Lougher <phillip@...ashfs.org.uk>
To: Justin Stitt <justinstitt@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] Squashfs: replace deprecated strncpy with strscpy

On 28/03/2024 21:52, 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()
> [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().
> 

Squashfs copies the passed string into a temporary buffer to ensure it
is NUL-terminated.  This however is completely unnecessary as the
string is already NUL-terminated.

A better way to remove the strncpy() is to remove the unnecessary string
copy, which I have done in this patch here:

https://lore.kernel.org/lkml/20240403183352.391308-1-phillip@squashfs.org.uk/T/#u

Phillip

> 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);
>   
>   	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>
>