linux-hardening - Re: [PATCH] CIFS: Fix a potencially linear read overflow

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Date:   Wed, 18 Aug 2021 09:21:57 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Len Baker <len.baker@....com>, Steve French <sfrench@...ba.org>,
        Suresh Jayaraman <sjayaraman@...e.de>
Cc:     linux-cifs@...r.kernel.org, samba-technical@...ts.samba.org,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] CIFS: Fix a potencially linear read overflow

On Tue, 2021-08-17 at 12:27 +0200, Len Baker 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.
> 
> Also, the strnlen() call does not avoid the read overflow in the strlcpy
> function when a not NUL-terminated string is passed.
> 
> So, replace this block by a call to kstrndup() that avoids this type of
> overflow and does the same.
> 
> Fixes: 066ce6899484d ("cifs: rename cifs_strlcpy_to_host and make it use new functions")
> Signed-off-by: Len Baker <len.baker@....com>
> ---
>  fs/cifs/cifs_unicode.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/cifs/cifs_unicode.c b/fs/cifs/cifs_unicode.c
> index 9bd03a231032..171ad8b42107 100644
> --- a/fs/cifs/cifs_unicode.c
> +++ b/fs/cifs/cifs_unicode.c
> @@ -358,14 +358,9 @@ cifs_strndup_from_utf16(const char *src, const int maxlen,
>  		if (!dst)
>  			return NULL;
>  		cifs_from_utf16(dst, (__le16 *) src, len, maxlen, codepage,
> -			       NO_MAP_UNI_RSVD);
> +				NO_MAP_UNI_RSVD);
>  	} else {
> -		len = strnlen(src, maxlen);
> -		len++;
> -		dst = kmalloc(len, GFP_KERNEL);
> -		if (!dst)
> -			return NULL;
> -		strlcpy(dst, src, len);
> +		dst = kstrndup(src, maxlen, GFP_KERNEL);
>  	}
> 
>  	return dst;
> --
> 2.25.1
> 

Reviewed-by: Jeff Layton <jlayton@...nel.org>