linux-kernel - Re: DoS with unprivileged mounts

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Date:	Thu, 15 Aug 2013 11:29:01 +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>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: DoS with unprivileged mounts

On Thu, Aug 15, 2013 at 8:45 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Part of me does prefer the semantics Andy has suggested where instead of
> unmounting things we have something like a skeleton of the mount tree
> unioned with dcaches of the filesystems themselves.  With "struct
> mountpoint" we are amazing close to that already.
>
> A mount skeleton would allow us to always remove and rename directories
> and files without really caring, about what mounts were present.
> Probably with just a quick lookup to see if we need to set
> DCACHE_MOUNTED.

Yes, we could have a separate dentry tree just for anchoring mounts
and we could make a union with the real dentry tree.  But implementing
that in a low overhead manner is not trivial.  Anchoring mounts on
real dentries is *rather* convenient.   And yes, we only actually need
the skeleton dentries when the real ones disappear, but that doesn't
make the implementation any simpler.

> The big practical problem I can see with MNT_VOLATILE is mount points in
> shared directories like /tmp but without the sticky set.  At which point
> it would be possible to delete another users mount points.  Perhaps we
> need restrictions on where a user can mount.

I think if user X mounts something on /tmp/foo owned by user Y and
user Y removes /tmp/foo then that shouldn't really surprise user X.  I
don't see this an issue.

Thanks,
Miklos
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