linux-kernel - Re: DoS with unprivileged mounts

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Date:	Wed, 14 Aug 2013 13:25:34 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: DoS with unprivileged mounts

On Wed, Aug 14, 2013 at 12:53 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Andy Lutomirski <luto@...capital.net> writes:
>
>> On 08/14/2013 10:42 AM, Miklos Szeredi wrote:
>>> There's a simple and effective way to prevent unlink(2) and rename(2)
>>> from operating on any file or directory by simply mounting something
>>> on it.  In any mount instance in any namespace.
>>>
>>> Was this considered in the unprivileged mount design?
>>>
>>> The solution is also theoretically simple: mounts in unpriv namespaces
>>> are marked "volatile" and are dissolved on an unlink type operation.
>>
>> I'd actually prefer the reverse: unprivileged mounts don't prevent
>> unlink and rename.  If the dentry goes away, then the mount could still
>> exist, sans underlying file.  (This is already supported on network
>> filesystems.)
>
> Of course we do this in network filesystems by pretending the
> rename/unlink did not actually happen.  The vfs insists that it be lied
> to instead of mirroring what actually happened.
>
> Again all of this is a question about efficient data structures and not
> really one of semantics.  Can either semantic be implemented in such a
> way that it does not slow down the vfs?

Given that vfs_unlink has:

	if (d_mountpoint(dentry))
		error = -EBUSY;

I think it's just a matter of changing / deleting that code.

--Andy
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