linux-hardening - Re: [PATCH v1 1/3] Fix unefficient call to memset before memcpu in nla

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Message-ID: <202010161618.8E214473@keescook>
Date:   Fri, 16 Oct 2020 16:19:59 -0700
From:   Kees Cook <keescook@...omium.org>
To:     laniel_francis@...vacyrequired.com
Cc:     linux-hardening@...r.kernel.org
Subject: Re: [PATCH v1 1/3] Fix unefficient call to memset before memcpu in
 nla_strlcpy.

On Fri, Oct 16, 2020 at 02:52:14PM +0200, laniel_francis@...vacyrequired.com wrote:
> From: Francis Laniel <laniel_francis@...vacyrequired.com>
> 
> This patch solves part 1 of issue: https://github.com/KSPP/linux/issues/110
> 
> Signed-off-by: Francis Laniel <laniel_francis@...vacyrequired.com>
> ---
>  lib/nlattr.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/nlattr.c b/lib/nlattr.c
> index 74019c8ebf6b..ab96a5f4b9b8 100644
> --- a/lib/nlattr.c
> +++ b/lib/nlattr.c
> @@ -731,8 +731,8 @@ size_t nla_strlcpy(char *dst, const struct nlattr *nla, size_t dstsize)
>  	if (dstsize > 0) {
>  		size_t len = (srclen >= dstsize) ? dstsize - 1 : srclen;
>  
> -		memset(dst, 0, dstsize);
>  		memcpy(dst, src, len);
> +		dst[len] = '\0';

I don't think this is right: callers are likely depending on the entire
destination buffer to be zero-padded. I think you probably want:

		memset(dst + len, 0, dstsize - len);
  		memcpy(dst, src, len);

(but double-check my pointer math)

>  	}
>  
>  	return srclen;
> -- 
> 2.20.1
> 

-- 
Kees Cook