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

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Sun, Sep 06, 2009 at 05:46:41PM +0300, Antti Kantee wrote:
> > > 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.

NetBSD-SA1999-006.txt.asc
NetBSD-SA1999-007.txt.asc

vfs.generic.usermount on Feb 16 2000 (vfs_syscalls.c rev 1.150).

> 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.

I think the discussion about this also refereced the above SA (which had been
fixed at this time, but showed how fragile usermounts are), but I may
be wrong.


> 
> > > 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)

I didn't say adding group-based usermounts was a good idea. I just said
that if it was done it should be sysctl'able :)

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--