linux-kernel - Re: [PATCH 1/2] netlink: Bounds-check nlmsg

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Message-ID: <202208312324.F2F8B28CA@keescook>
Date:   Wed, 31 Aug 2022 23:27:08 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        syzbot <syzkaller@...glegroups.com>,
        Yajun Deng <yajun.deng@...ux.dev>, netdev@...r.kernel.org,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/2] netlink: Bounds-check nlmsg_len()

On Wed, Aug 31, 2022 at 08:18:25PM -0700, Jakub Kicinski wrote:
> On Wed, 31 Aug 2022 20:06:09 -0700 Kees Cook wrote:
> >  static inline int nlmsg_len(const struct nlmsghdr *nlh)
> >  {
> > -	return nlh->nlmsg_len - NLMSG_HDRLEN;
> > +	u32 nlmsg_contents_len;
> > +
> > +	if (WARN_ON_ONCE(check_sub_overflow(nlh->nlmsg_len,
> > +					    (u32)NLMSG_HDRLEN,
> > +					    &nlmsg_contents_len)))
> > +		return 0;
> > +	if (WARN_ON_ONCE(nlmsg_contents_len > INT_MAX))
> > +		return INT_MAX;
> > +	return nlmsg_contents_len;
> 
> We check the messages on input, making sure the length is valid wrt
> skb->len, and sane, ie > NLMSG_HDRLEN. See netlink_rcv_skb().
> 
> Can we not, pretty please? :(

This would catch corrupted values...

Is the concern the growth in image size? The check_sub_overflow() isn't
large at all -- it's just adding a single overflow bit test. The WARNs
are heavier, but they're all out-of-line.

-- 
Kees Cook