linux-kernel - Grrrr fusermount.

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Message-ID: <8761t65kd2.fsf_-_@xmission.com>
Date:	Wed, 09 Oct 2013 12:12:41 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Miklos Szeredi <miklos@...redi.hu>
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>,
	Rob Landley <rob@...dley.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Grrrr fusermount.

ebiederm@...ssion.com (Eric W. Biederman) writes:

> But I will go through and read the old fusermount code before I get too
> much farther just so I understand what I am potentially breaking.

Grr.

So I have just read the fusermount umount code and the hack that it uses
before there was UMOUNT_NOFOLLOW support in the vm.

If I walk this path of lazy unmounts and detaching directories, anyone
with a new kernel and an old copy of fusermount and a nfs mounted home
directory will be able to exploit the fusermount umount symlink race.

Unless we can declare that old fusermount binaries are buggy beyond
supporting this patchset as it exists is dead.

Eric
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