linux-kernel - Re: Thoughts on credential switching

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Message-ID: <20140327185648.GE2526@jeremy-laptop>
Date:	Thu, 27 Mar 2014 11:56:48 -0700
From:	Jeremy Allison <jra@...ba.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Jeremy Allison <jra@...ba.org>, Jeff Layton <jlayton@...hat.com>,
	Florian Weimer <fweimer@...hat.com>,
	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, Mar 27, 2014 at 11:46:39AM -0700, Andy Lutomirski wrote:
> On Thu, Mar 27, 2014 at 11:26 AM, Jeremy Allison <jra@...ba.org> wrote:
> >
> > Amen to that :-).
> >
> > However, after talking with Jeff and Jim at CollabSummit,
> > I was 'encouraged' to make my opinions known on the list.
> >
> > To me, calling the creds handle a file descriptor just
> > feels wrong. IT *isn't* an fd, you can't read/write/poll
> > on it, and it's only done as a convenience to get the
> > close-on-exec semantics and the fact that the creds are
> > already hung off the fd's in kernel space.
> 
> Windows calls these things "handles."  Linux has "file descriptors,"
> and there's plenty of precedent for things that aren't files.

Sure, but there's a set of expectations around
fd's that these things don't satisfy - IO-ops.

> > That way we can also make it clear this thing only has
> > meaning to a thread group, and SHOULD NOT (and indeed
> > preferably CAN NOT) be passed between processes.
> >
> 
> If you want those semantics, then stick a struct pid * in there for
> the tgid of the cretor and make sure that current's tgid matches when
> you try to use it.
> 
> I think they'd be more useful without that check, though.

I'm more worried about leakage and unintended consequences
here.

> BTW, what do you want to have happen on fork?  I think they should keep working.

Yeah, that's true. I want them to keep
working across fork, but not across exec
or any other method of fd-passing.

Jeremy.
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