linux-kernel - Re: DoS with unprivileged mounts

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Message-ID: <CAJfpeguv+7giYNpAuXE9Ja_9BEwB0-fZBVgRSeVqpzSXgQYZ6Q@mail.gmail.com>
Date:	Thu, 15 Aug 2013 06:59:59 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"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 9:32 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:

>> The solution is also theoretically simple: mounts in unpriv namespaces
>> are marked "volatile" and are dissolved on an unlink type operation.
>>
>> Such volatile mounts would be useful in general too.
>
> Agreed.
>
> This is a problem that is a general pain with mount namespaces in
> general.
>
> I think the real technical hurdle is finding the mounts t in some random
> mount namespace.  Once we can do that relatively efficiently the rest
> becomes simple.

We already have a "struct mountpoint" hashed on the dentry.  Chaining
mounts on that mountpoint would be trivial.  And we need a
MNT_VOLATILE flag and that's it.  If we fear that traversing the list
of mounts on the dentry to check for non-volatile ones then we could
also add a separate volatile counter to struct mountpoint and a
matching flag to the dentry.  But I don't think that's really
necessary.

Thanks,
Miklos
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