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

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Message-ID: <CAHC9VhT2-QJ6yRoAvbicg5n_NUZLpJ5YjNer4TcHwiaW2hq6FQ@mail.gmail.com>
Date:   Mon, 13 Mar 2023 16:43:35 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.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 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).

Regardless, it looks fine to me.

Acked-by: Paul Moore <paul@...l-moore.com>

> > 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

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