Re: [PATCH] scm: fold __scm_send() into scm_send()

[Andrew Morton]

Chris Wright <chrisw@xxxxxxxxxxxx> wrote:
>
> * Chris Wright (chrisw@xxxxxxxxxxxx) wrote:
> > * Ingo Oeser (netdev@xxxxxxxx) wrote:
> > > Hi Chris,
> > > 
> > > Andrew Morton wrote:
> > > > Ingo Oeser <ioe-lkml@xxxxxxxxxx> wrote:
> > > > >
> > > > >  -int scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *scm)
> > > > >  -{
> > > > >  -	struct task_struct *p = current;
> > > > >  -	scm->creds = (struct ucred) {
> > > > >  -		.uid = p->uid,
> > > > >  -		.gid = p->gid,
> > > > >  -		.pid = p->tgid
> > > > >  -	};
> > > > >  -	scm->fp = NULL;
> > > > >  -	scm->sid = security_sk_sid(sock->sk, NULL, 0);
> > > > >  -	scm->seq = 0;
> > > > >  -	if (msg->msg_controllen <= 0)
> > > > >  -		return 0;
> > > > >  -	return __scm_send(sock, msg, scm);
> > > > >  -}
> > > > 
> > > > It's worth noting that scm_send() will call security_sk_sid() even if
> > > > (msg->msg_controllen <= 0).
> > > 
> > > Chris, do you know if this is needed in this case?
> > 
> > This whole thing is looking broken.  I'm still trying to find the original
> > patch which caused the series of broken patches on top.
> 
> OK, it starts here from Catherine's patch:
> 
> include/net/scm.h::scm_recv()
> +	if (test_bit(SOCK_PASSSEC, &sock->flags)) {
> +		err = security_sid_to_context(scm->sid, &scontext, &scontext_len);
> +		if (!err)
> + 			put_cmsg(msg, SOL_SOCKET, SCM_SECURITY, scontext_len, scontext);
> + }
> 
> Catherine, the security_sid_to_context() is a raw SELinux function which
> crept into core code and should not have been there.  The fallout fixes
> included conditionally exporting security_sid_to_context, and finally
> scm_send/recv unlining.

Yes.  So we're OK up the uninlining, right?

>  The end result in -mm looks broken to me.
> Specifically, it now does:
> 
> 	ucred->uid = tsk->uid;
> 	ucred->gid = tsk->gid;
> 	ucred->pid = tsk->tgid;
> 	scm->fp = NULL;
> 	scm->seq = 0;
> 	if (msg->msg_controllen <= 0)
> 		return 0;
> 
> 	scm->sid = security_sk_sid(sock->sk, NULL, 0);
> 
> The point of Catherine's original patch was to make sure there's always
> a security identifier associated with AF_UNIX messages.  So receiver
> can always check it (same as having credentials even w/out sender
> control message passing them).  Now we will have garbage for sid.

This answers the question I've been asking all and sundry for a week, thanks ;)

So:

- scm-fold-__scm_send-into-scm_send.patch is OK

- scm_send-speedup.patch is wrong

- Catherine's patch introduces a possibly-significant performance
  problem: we're now calling the expensive-on-SELinux security_sk_sid()
  more frequently than we used to.

- That "initialise scm->creds via a temporary struct" trick still
  generates bad code.


I actually have enough to be going on with here - I'll drop it all.
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