linux-kernel - Re: [PATCH net-next v2] scm: fix MSG_CTRUNC setting condition for SO

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Message-ID: <CAEivzxcbp61xdDL6mfoMBu4t5C3auyDO_-ec7wHu0EbN=zh2WQ@mail.gmail.com>
Date:   Tue, 14 Mar 2023 12:06:46 +0100
From:   Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To:     Paul Moore <paul@...l-moore.com>
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>,
        Leon Romanovsky <leon@...nel.org>, jmorris@...ei.org,
        serge@...lyn.com, linux-security-module@...r.kernel.org,
        selinux@...r.kernel.org
Subject: Re: [PATCH net-next v2] scm: fix MSG_CTRUNC setting condition for SO_PASSSEC

On Mon, Mar 13, 2023 at 9:43 PM Paul Moore <paul@...l-moore.com> wrote:
>
> On Mon, Mar 13, 2023 at 7:40 AM Aleksandr Mikhalitsyn
> <aleksandr.mikhalitsyn@...onical.com> wrote:
> >
> > +CC security subsystem folks
> >
> > On Mon, Mar 13, 2023 at 12:32 PM Alexander Mikhalitsyn
> > <aleksandr.mikhalitsyn@...onical.com> wrote:
> > >
> > > Currently, kernel would set MSG_CTRUNC flag if msg_control buffer
> > > wasn't provided and SO_PASSCRED was set or if there was pending SCM_RIGHTS.
> > >
> > > For some reason we have no corresponding check for SO_PASSSEC.
> > >
> > > In the recvmsg(2) doc we have:
> > >        MSG_CTRUNC
> > >               indicates that some control data was discarded due to lack
> > >               of space in the buffer for ancillary data.
> > >
> > > So, we need to set MSG_CTRUNC flag for all types of SCM.
> > >
> > > This change can break applications those don't check MSG_CTRUNC flag.
>
> Unless I'm missing something I don't think this will actually result
> in a userspace visible change as put_cmsg() already has a number of
> checks which set the MSG_CTRUNC flag if necessary (including if no
> control buffer is passed, e.g. msg_control == NULL).

Yes you are right. I found this check suspicious while working on
SCM_PIDFD (which is not yet submitted to LKML),
I think it is worth fixing that check anyway just for consistency reasons.

>
> Regardless, it looks fine to me.
>
> Acked-by: Paul Moore <paul@...l-moore.com>

Thanks, Paul!

Regards,
Alex

>
> > > 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>
> > > Cc: Leon Romanovsky <leon@...nel.org>
> > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
> > >
> > > v2:
> > > - commit message was rewritten according to Eric's suggestion
> > > ---
> > >  include/net/scm.h | 13 ++++++++++++-
> > >  1 file changed, 12 insertions(+), 1 deletion(-)
> > >
> > > 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 */
> > >
> > >  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
>
> --
> paul-moore.com