netdev - Re: [PATCH][next] net: dev: Convert sa_data to flexible array in struct sockaddr

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Message-ID: <202210211841.031AB46@keescook>
Date:   Fri, 21 Oct 2022 18:52:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Dylan Yudaken <dylany@...com>,
        Yajun Deng <yajun.deng@...ux.dev>,
        Petr Machata <petrm@...dia.com>,
        Hangbin Liu <liuhangbin@...il.com>,
        Leon Romanovsky <leon@...nel.org>,
        syzbot <syzkaller@...glegroups.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        netdev@...r.kernel.org, Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        "D. Wythe" <alibuda@...ux.alibaba.com>,
        Jeremy Kerr <jk@...econstruct.com.au>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Menglong Dong <imagedong@...cent.com>,
        Kuniyuki Iwashima <kuniyu@...zon.com>,
        Congyu Liu <liu3101@...due.edu>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Ziyang Xuan <william.xuanziyang@...wei.com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] net: dev: Convert sa_data to flexible array in
 struct sockaddr

On Thu, Oct 20, 2022 at 10:58:50AM +0200, Paolo Abeni wrote:
> On Tue, 2022-10-18 at 02:56 -0700, Kees Cook wrote:
> > [...]
> >  struct sockaddr {
> >  	sa_family_t	sa_family;	/* address family, AF_xxx	*/
> > -	char		sa_data[14];	/* 14 bytes of protocol address	*/
> > +	union {
> > +		char sa_data_min[14];		/* Minimum 14 bytes of protocol address	*/
> > +		DECLARE_FLEX_ARRAY(char, sa_data);
> 
> Any special reason to avoid preserving the old name for the array and
> e.g. using sa_data_flex for the new field, so we don't have to touch
> the sockaddr users?

Yes -- the reason is exactly to not touch the sockaddr users (who
generally treat sa_data as a fake flexible array). By switching it to a
flex-array the behavior will stay the same (especially under the coming
-fstrict-flex-arrays option), except that it breaks sizeof(). But the
broken sizeof() allows us to immediately find all the places where the
code explicitly depends on sa_data being 14 bytes. And for those cases,
we switch to sizeof(sa_data_min).

If we went the reverse route (and added -fstrict-flex-arrays) we might
end up adding a bunch of false positives all at once, because the places
that treated it as a flex-array would suddenly all begin behaving as a
14-byte array.

-- 
Kees Cook