From mboxrd@z Thu Jan 1 00:00:00 1970 From: Florian Weimer Subject: Re: Thoughts on credential switching Date: Thu, 27 Mar 2014 14:06:32 +0100 Message-ID: <53342258.8000304@redhat.com> References: <53341D8E.80105@redhat.com> <20140327060225.4f4caa5a@ipyr.poochiereds.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Andy Lutomirski , Jim Lieb , "Eric W. Biederman" , LSM List , "Serge E. Hallyn" , Kees Cook , Linux FS Devel , "Theodore Ts'o" , "linux-kernel@vger.kernel.org" , bfields@redhat.com To: Jeff Layton Return-path: In-Reply-To: <20140327060225.4f4caa5a@ipyr.poochiereds.net> Sender: linux-security-module-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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