Re: [PATCH v1 1/3] Fix unefficient call to memset before memcpu in nla_strlcpy.

[Francis Laniel <laniel_francis@privacyrequired.com>]

Le samedi 17 octobre 2020, 01:19:59 CEST Kees Cook a écrit :
> On Fri, Oct 16, 2020 at 02:52:14PM +0200, laniel_francis@privacyrequired.com 
wrote:
> > From: Francis Laniel <laniel_francis@privacyrequired.com>
> > 
> > This patch solves part 1 of issue:
> > https://github.com/KSPP/linux/issues/110
> > 
> > Signed-off-by: Francis Laniel <laniel_francis@privacyrequired.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)
> 

I did not understand that the content has to be zero-padded, now your comment 
on github is clearer!
I will modifiy it for next version and check the math on the paper.

> >  	}
> >  	
> >  	return srclen;