linux-kernel - Re: [PATCH] umount: Do not allow unmounting rootfs.

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Date:	Tue, 07 Oct 2014 16:35:10 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Vagin <avagin@...allels.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Andrey Vagin <avagin@...nvz.org>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Serge Hallyn <serge.hallyn@...onical.com>
Subject: Re: [PATCH] umount:  Do not allow unmounting rootfs.

Andrew Vagin <avagin@...allels.com> writes:

> On Tue, Oct 07, 2014 at 01:58:01PM -0700, Eric W. Biederman wrote:
>> Andrew Vagin <avagin@...allels.com> writes:
>> 
>> > On Tue, Oct 07, 2014 at 12:27:06PM -0700, Eric W. Biederman wrote:
>> >> 
>> >> Which in practice is totally uninteresting.  Only the global root user can
>> >> do it, and it is just a stupid thing to do.
>> >> 
>> >> However that is no excuse to allow a silly way to oops the kernel.
>> >> 
>> >> We can avoid this silly problem by setting MNT_LOCKED on the rootfs
>> >> mount point and thus avoid needing any special cases in the unmount
>> >> code.
>> >
>> > I had this idea too, but it doesn't work.
>> >
>> > MNT_LOCKED isn't inherited, if the privileged user creates a new mount
>> > namespace.
>> >
>> > So "unshame -m ./nsenter" reproduces the same BUG.
>> 
>> Which broken tree do you have where MNT_LOCKED is not inherited?
>
> It is Linus' tree with your patch.
>
> I commented out one line and the BUG isn't triggered any more.

Ok.  That is very very weird.  It works for me and not for you.

Doh!  I ran your test program first on the primary mount namespace
and didn't have /proc mounted when I tried to run your test program
later.

Thanks for the hint about copy_tree that does seem to be where the logic
goes haywire.  In copy_mnt_ns we want an exact copy not a partial copy.
The good news is that this bug could only affect rootfs, so it is not a
security hole with respect to mount namespaces, created with user
namespace permissions.

mount --rbind and mount propogation should not be locked to their parent
mounts so we do need to clear MNT_LOCKED in a few places.

A patch that fixes copy_tree carefully in a moment.

Eric
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