Re: mount(2) in 2.3.99pre9!!!

From: Alexander Viro (
Date: Thu Jun 01 2000 - 04:13:56 EST

On Wed, 31 May 2000, Chris Evans wrote:

> Hi,
> I notice that in 2.3.99pre9 and newer, the system call mount(2) will under
> some circumstances allow unprivileged users to mount things.

Yes, it does.

> Can anyone elaborate on what precisely we allow and why? Despite being
> named "do_loopback", it looks like a simple aliasing mechanism.

Yes, it is, and yes, it happened to be simple after some work ;-)

> At the very least, mount_is_safe() would seem to be missing a check for
> write permission on the parent directory of the mount-point.

Hmm... What's the problem with situation when you have write permissions
on mountpoint but not on its parent? MAY_WRITE on mountpoint is checked,
unless I've really fscked up. Comments on security implications are more
than welcome - the variant I've done was, basically, "can we reach the
thing to be mounted anyway and can we already do whatever we want with the
contents of the mountpoint?"

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