linux-hardening - Re: [RFC][PATCH v2 2/3] Modify return value of nla

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Date:   Mon, 19 Oct 2020 16:01:27 -0700
From:   Kees Cook <keescook@...omium.org>
To:     laniel_francis@...vacyrequired.com
Cc:     Jakub Kicinski <kuba@...nel.org>, linux-hardening@...r.kernel.org,
        davem@...emloft.net
Subject: Re: [RFC][PATCH v2 2/3] Modify return value of nla_strlcpy to match
 that of strscpy.

On Mon, Oct 19, 2020 at 09:43:55AM -0700, Jakub Kicinski wrote:
> On Mon, 19 Oct 2020 17:23:30 +0200 laniel_francis@...vacyrequired.com
> wrote:
> > -size_t nla_strlcpy(char *dst, const struct nlattr *nla, size_t dstsize)
> > +ssize_t nla_strlcpy(char *dst, const struct nlattr *nla, size_t dstsize)
> >  {
> > +	size_t len;
> > +	ssize_t ret;
> >  	size_t srclen = nla_len(nla);
> >  	char *src = nla_data(nla);
> 
> Sort local variables long to short.

Specifically, "reverse christmas tree":

 	size_t srclen = nla_len(nla);
 	char *src = nla_data(nla);
 	size_t len;
 	ssize_t ret;


> > diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
> > index 41a55c6cbeb8..f0bf64393cbf 100644
> > --- a/net/sched/cls_api.c
> > +++ b/net/sched/cls_api.c
> > @@ -223,7 +223,7 @@ static inline u32 tcf_auto_prio(struct tcf_proto *tp)
> >  static bool tcf_proto_check_kind(struct nlattr *kind, char *name)
> >  {
> >  	if (kind)
> > -		return nla_strlcpy(name, kind, IFNAMSIZ) >= IFNAMSIZ;
> > +		return nla_strlcpy(name, kind, IFNAMSIZ) > 0;
> 
> Bug.
> 
> >  	memset(name, 0, IFNAMSIZ);
> >  	return false;
> >  }

Have you been able to exercise the changed code paths? (I would have
expected this to immediately fail, for example.)

-- 
Kees Cook