linux-kernel - Re: [patch 5/8] allow unprivileged bind mounts

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Message-Id: <E1HfWML-00048U-00@dorka.pomaz.szeredi.hu>
Date:	Sun, 22 Apr 2007 09:19:45 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	ebiederm@...ssion.com
CC:	miklos@...redi.hu, akpm@...ux-foundation.org, serue@...ibm.com,
	viro@....linux.org.uk, linuxram@...ibm.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	containers@...ts.osdl.org
Subject: Re: [patch 5/8] allow unprivileged bind mounts

> > From: Miklos Szeredi <mszeredi@...e.cz>
> >
> > Allow bind mounts to unprivileged users if the following conditions
> > are met:
> >
> >   - mountpoint is not a symlink or special file
> 
> Why?  This sounds like a left over from when we were checking permissions.

Hmm, yes.  Don't know.  Maybe only the symlink check.

Bind mounts of directory over non-directy, and vica versa are already
excluded, even for root.

> >   - parent mount is owned by the user
> >   - the number of user mounts is below the maximum
> >
> > Unprivileged mounts imply MS_SETUSER, and will also have the "nosuid"
> > and "nodev" mount flags set.
> 
> So in principle I agree, but in detail I disagree.
> 
> capable(CAP_SETUID) should be required to leave MNT_NOSUID clear.
> capable(CAP_MKNOD) should be required to leave MNT_NODEV clear.
> 
> I.e.  We should not special case this as a user mount but rather
> simply check to see if the user performing the mount has the appropriate
> capabilities to allow the flags.

Sounds sane.  Will fix.

Miklos
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