linux-kernel - Re: Thoughts on credential switching

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Date:	Thu, 27 Mar 2014 06:02:24 -0700
From:	Jeff Layton <jlayton@...hat.com>
To:	Florian Weimer <fweimer@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Jim Lieb <jlieb@...asas.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	LSM List <linux-security-module@...r.kernel.org>,
	"Serge E. Hallyn" <serge@...onical.com>,
	Kees Cook <keescook@...omium.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	bfields@...hat.com
Subject: Re: Thoughts on credential switching

On Thu, 27 Mar 2014 13:46:06 +0100
Florian Weimer <fweimer@...hat.com> wrote:

> On 03/27/2014 01:23 AM, Andy Lutomirski wrote:
> 
> > I propose the following set of new syscalls:
> >
> > int credfd_create(unsigned int flags): returns a new credfd that
> > corresponds to current's creds.
> >
> > int credfd_activate(int fd, unsigned int flags): Change current's
> > creds to match the creds stored in fd.  To be clear, this changes
> > both the "subjective" and "objective" (aka real_cred and cred)
> > because there aren't any real semantics for what happens when
> > userspace code runs with real_cred != cred.
> 
> This interface does not address the long-term lack of POSIX
> compliance in setuid and friends, which are required to be
> process-global and not thread-specific (as they are on the kernel
> side).
> 
> glibc works around this by reserving a signal and running set*id on 
> every thread in a special signal handler.  This is just crass, and it
> is likely impossible to restore the original process state in case of 
> partial failure.  We really need kernel support to perform the 
> process-wide switch in an all-or-nothing manner.
> 

I disagree. We're treading new ground here with this syscall. It's
not defined by POSIX so we're under no obligation to follow its silly
directives in this regard. Per-process cred switching doesn't really
make much sense in this day and age, IMO. Wasn't part of the spec was
written before threading existed


The per-process credential switching is pretty universally a pain in
the ass for anyone who wants to write something like a threaded file
server. Jeremy Allison had to jump through some rather major hoops to
work around it for Samba [1]. I think we want to strive to make this a
per-task credential switch and ignore that part of the POSIX spec.

That said, I think we will need to understand and document what we
expect to occur if someone does this new switch_creds(fd) call and then
subsequently calls something like setuid(), if only to ensure that we
don't get blindsided by it.

[1]: http://sourceware.org/ml/libc-help/2012-07/msg00004.html
-- 
Jeff Layton <jlayton@...hat.com>
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