Re: kern/29360: vfs.generic.usermount and mount(8) general questions

[Antti Kantee <pooka%cs.hut.fi@localhost>]

The following reply was made to PR kern/29360; it has been noted by GNATS.

From: Antti Kantee <pooka%cs.hut.fi@localhost>
To: Manuel Bouyer <bouyer%antioche.eu.org@localhost>
Cc: Elad Efrat <elad%NetBSD.org@localhost>, gnats-bugs%NetBSD.org@localhost,
        tech-kern%NetBSD.org@localhost
Subject: Re: kern/29360: vfs.generic.usermount and mount(8) general questions
Date: Sun, 6 Sep 2009 17:46:41 +0300

 On Sun Sep 06 2009 at 11:20:09 +0200, Manuel Bouyer wrote:
 > On Sun, Sep 06, 2009 at 02:01:44AM -0400, Elad Efrat wrote:
 > > Hi,
 > > 
 > > I just came across this PR.
 > > 
 > > The check that a non-root user owns the mount-point directory was
 > > introduced way before vfs.generic.usermount. In fact, it seems that it
 > > actually removed the root check, and allowed non-root users to freely
 > > mount file-systems:
 > > 
 > >     
 > > http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/kern/vfs_syscalls.c.diff?r1=1.42&r2=1.43&f=h
 > 
 > Yes, vfs.generic.usermount was introduced later, because of security issues
 > that usermounts could cause. AFAIK the know security issues with
 > usermounts are fixed, but still it's better to have it disabled on systems
 > where it's not needed.
 
 Really?  If you are going to claim a fixed security issue, please provide
 some reference to the issue you are talking about.  As I recall, it was
 added because mounting enough file systems (I used kernfs for testing back
 then) would cause the kernel to run out of memory and the system to panic.
 
 > > With something like the following:
 > > 
 > > /* Ensure that the user can write to the mount-point. */
 > > if ((error = VOP_ACCESS(vp, VWRITE, l->l_cred)) != 0)
 > >     return error;
 > > 
 > > Does anyone see any drawbacks to this approach? If not, I'll change
 > > the relevant code.
 > 
 > Yes, that would mean a user could mount his own FS over e.g. /tmp, or
 > /var/mail. that's bad.
 > I think that checking the user owns the mount point is the right thing to do.
 
 I agree that ownership is the right check.
 
 > I think a sysctl to control whenever to check for group ownerchip instead
 > of user ownerchip would work, though. It's up to the admin to carefully
 > choose a group for devices and mount points :)
 
 I am opposed to adding a kernel switch with confusing security
 implications.  Especially since the issue in the PR is corner-case (IMHO,
 of course) and can be solved easily at user-level with a wrapper without
 affecting everyone.
 
 (at the very least, you'd need to check owner || (group && write).
 and even then, there are difficult-to-foresee consequences e.g. a
 sticky-bitty group-shared working directory or group +wx "drop site"
 directory)