linux-kernel - Regression introduced by commit dbe9a4173ea53

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Message-ID: <20150123141623.GK30451@lnxjonasdn.se.axis.com>
Date:	Fri, 23 Jan 2015 15:16:23 +0100
From:	Jonas Danielsson <jonas.danielsson@...s.com>
To:	<davem@...emloft.net>
CC:	<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
	<ebiederm@...ssion.com>, <luto@...capital.net>, <rgb@...hat.com>,
	<johannes.berg@...el.com>, <viro@...iv.linux.org.uk>,
	<akpm@...ux-foundation.org>, <fabf@...net.be>,
	<brian.cambell@...tshare.com>, <mpatocka@...hat.com>,
	<tgraf@...g.ch>, <eric.dumazet@...il.com>, <desrt@...rt.ca>
Subject: Regression introduced by commit dbe9a4173ea53

Hi,

I have seen a regression, that I think is caused by:

commit dbe9a4173ea53b72b2c35d19f676a85b69f1c9fe
Author: Eric W. Biederman <ebiederm@...ssion.com>
Date:   Thu Sep 6 18:20:01 2012 +0000

    scm: Don't use struct ucred in NETLINK_CB and struct scm_cookie.

With this commit the value send as uid when credentials are missing
changes from -1 to overflowuid (default -2, the 'nobody' user).

I was using dbus-send to perform a method call on a gdbus server.
And sometimes I fall victim to the race condition caused by:

commit 16e5726 ("af_unix: dont send SCM_CREDENTIALS by default")

And then there will be no credentials on the socket. The glib library checks
the uid, and if it is -1, it will fall back to using SO_PEERCRED and things
will work for me. But since the commit in $subject changes the uid I get
failure with NoReply from dbus-send.

It seems that before the commit in $subject that the function from_kuid_munged
was only called if there were credentials present. Otherwise we would set uid
to -1. Now uid gets set to -1 if there is no credentials but from_kuid_munged
is always called. And from the documentation of from_kuid_munged it states
that it 'never fails and always returns a valid uid'. And in the case of
uid being -1, it returns overflowuid.

So this seems like it broke glib. Caused by 1) the introduced race condition
that makes it not totally safe to set SO_PEERCRED on the accepted socket and
2) that the value of uid when credentials are missing changed from -1 to
overflowuid.

Thanks for your time
Jonas

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