linux-kernel - Re: [PATCH net-next] scm: fix MSG_CTRUNC setting condition for SO

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Message-ID: <CAEivzxeorZoiE4VmJ45CoF4ZRoW3B+rkT0ufX7y1bxn510yzPQ@mail.gmail.com>
Date:   Mon, 27 Feb 2023 10:55:04 +0100
From:   Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     davem@...emloft.net, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next] scm: fix MSG_CTRUNC setting condition for SO_PASSSEC

On Mon, Feb 27, 2023 at 10:47 AM Leon Romanovsky <leon@...nel.org> wrote:
>
> On Sun, Feb 26, 2023 at 09:17:30PM +0100, Alexander Mikhalitsyn wrote:
> > Currently, we set MSG_CTRUNC flag is we have no
> > msg_control buffer provided and SO_PASSCRED is set
> > or if we have pending SCM_RIGHTS.
> >
> > For some reason we have no corresponding check for
> > SO_PASSSEC.
> >
> > Cc: "David S. Miller" <davem@...emloft.net>
> > Cc: Eric Dumazet <edumazet@...gle.com>
> > Cc: Jakub Kicinski <kuba@...nel.org>
> > Cc: Paolo Abeni <pabeni@...hat.com>
> > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
> > ---
> >  include/net/scm.h | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
>
> Is it a bugfix? If yes, it needs Fixes line.

It's from 1da177e4c3 ("Linux-2.6.12-rc2") times :)
I wasn't sure that it's correct to put the "Fixes" tag on such an old
and big commit. Will do. Thanks!

>
> >
> > diff --git a/include/net/scm.h b/include/net/scm.h
> > index 1ce365f4c256..585adc1346bd 100644
> > --- a/include/net/scm.h
> > +++ b/include/net/scm.h
> > @@ -105,16 +105,27 @@ static inline void scm_passec(struct socket *sock, struct msghdr *msg, struct sc
> >               }
> >       }
> >  }
> > +
> > +static inline bool scm_has_secdata(struct socket *sock)
> > +{
> > +     return test_bit(SOCK_PASSSEC, &sock->flags);
> > +}
> >  #else
> >  static inline void scm_passec(struct socket *sock, struct msghdr *msg, struct scm_cookie *scm)
> >  { }
> > +
> > +static inline bool scm_has_secdata(struct socket *sock)
> > +{
> > +     return false;
> > +}
> >  #endif /* CONFIG_SECURITY_NETWORK */
>
> There is no need in this ifdef, just test bit directly.

The problem is that even if the kernel is compiled without
CONFIG_SECURITY_NETWORK
userspace can still set the SO_PASSSEC option. IMHO it's better not to
set MSG_CTRUNC
if CONFIG_SECURITY_NETWORK is disabled, msg_control is not set but
SO_PASSSEC is enabled.
Because in this case SCM_SECURITY will never be sent. Please correct
me if I'm wrong.

Kind regards,
Alex

>
> >
> >  static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
> >                               struct scm_cookie *scm, int flags)
> >  {
> >       if (!msg->msg_control) {
> > -             if (test_bit(SOCK_PASSCRED, &sock->flags) || scm->fp)
> > +             if (test_bit(SOCK_PASSCRED, &sock->flags) || scm->fp ||
> > +                 scm_has_secdata(sock))
> >                       msg->msg_flags |= MSG_CTRUNC;
> >               scm_destroy(scm);
> >               return;
> > --
> > 2.34.1
> >