linux-kernel - Re: Thoughts on credential switching

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Date:	Thu, 27 Mar 2014 14:06:32 +0100
From:	Florian Weimer <fweimer@...hat.com>
To:	Jeff Layton <jlayton@...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 03/27/2014 02:02 PM, Jeff Layton wrote:

>> 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

Okay, then we need to add a separate set of system calls.

I really, really want to get rid of that signal handler mess in glibc, 
with its partial failures.

> 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.

Yeah, I get that, setfsuid/setfsgid already behaves this way.

(Current directory and umask are equally problematic, but it's possible 
to avoid most issues.)

> 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.

Currently, from the kernel perspective, this is not really a problem 
because the credentials are always per-task.  It's just that a 
conforming user space needs the process-wide credentials.

-- 
Florian Weimer / Red Hat Product Security Team
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