linux-kernel - Re: [patch] unprivileged mounts update

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Message-ID: <20070425185244.GA20688@vino.hallyn.com>
Date:	Wed, 25 Apr 2007 13:52:44 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Miklos Szeredi <miklos@...redi.hu>, akpm@...ux-foundation.org,
	viro@....linux.org.uk, linuxram@...ibm.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	containers@...ts.osdl.org, linux-security-module@...r.kernel.org
Subject: Re: [patch] unprivileged mounts update

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> "Serge E. Hallyn" <serge@...lyn.com> writes:
> 
> > Quoting Eric W. Biederman (ebiederm@...ssion.com):
> >> 
> >> Are there other permission checks that mount is doing that we
> >> care about.
> >
> > Not mount itself, but in looking up /share/fa/root/home/fa,
> > user fa doesn't have the rights to read /share, and by setting
> > fsuid to fa and dropping CAP_DAC_READ_SEARCH the mount action fails.
> 
> Got it. 
> 
> I'm not certain this is actually a problem it may be a feature.
> But it does fly in the face of the general principle of just
> getting out of roots way so things can get done.
> 
> I think we can solve your basic problem by simply doing like:
> chdir(/share); mount(.);  To simply avoid the permission problem.
> 
> The practical question is how much do we care.
> 
> > But the solution you outlined in your previous post would work around
> > this perfectly.
> 
> If we are not using usual permissions which user do we use current->uid?
> Or do we pass that user someplace?

Right, I figure if the normal action is to always do
mnt->user = current->fsuid, then for the special case we
pass a uid in someplace.  Of course...  do we not have a
place to do that?  Would it be a no-no to use 'data' for
a non-fs-specific arg?

> >> > If it were really the equivalent then I could keep my capabilities :)
> >> > after changing it.
> >> 
> >> We drop all capabilities after we change the euid.
> >
> > Not if we've done prctl(PR_SET_KEEPCAPS, 1)
> 
> Ah cap_clear doesn't do the obvious thing.
> 
> Eric
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