From mboxrd@z Thu Jan  1 00:00:00 1970
From: Miklos Szeredi <miklos@szeredi.hu>
Subject: Re: [PATCH review 0/4] Loopback mount escape fixes
Date: Mon, 13 Apr 2015 14:18:40 +0200
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Cc: Linux Containers <containers@lists.linux-foundation.org>,
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To: "Eric W. Biederman" <ebiederm@xmission.com>
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On Thu, Apr 9, 2015 at 1:31 AM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> After the last round of feedback I sat down and played with my fix
> for the fact that a strategically placed rename, ".." on bind mounts
> go up past the root of the bind mount.
>
> The code better handles the escaped directory returning into it's bind
> mount, and is now roughly a constant factor cost in all cases from what
> the code costs without the fix.
>
> So I think I have found a better tradeoff between fixing this bug and
> not slowing down path name lookups in the common case.

Maybe I'm missing something, but I see a much simpler fix:

 - When following ".." first just check against the dentry being equal
to the root dentry.

 - If so, then check mount being equal to root mount.

 - If so, then we are fine, found the root.

 - If mount is not root mount, then we either have a bind mount or the
escape scenario. So have a peek at the mount tree to see if we have a
chance of reaching root or not.

  - If yes, then we are fine, continue upward.

  - Otherwise stop here and act like we found root.

This doesn't have to hook into d_move() and will only trigger the
"violated" mode on an very specific and rare case.

I haven't thought about this very hard, but I don't see how the root
dentry could be avoided without first having access to something
outside the root.

Thanks,
Miklos