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From: Andy Lutomirski <luto@amacapital.net>
To: Jann Horn <jannh@google.com>
Cc: cyphar@cyphar.com, "Eric W. Biederman" <ebiederm@xmission.com>,
	jlayton@kernel.org, Bruce Fields <bfields@fieldses.org>,
	Al Viro <viro@zeniv.linux.org.uk>, Arnd Bergmann <arnd@arndb.de>,
	shuah@kernel.org, David Howells <dhowells@redhat.com>,
	Andy Lutomirski <luto@kernel.org>,
	christian@brauner.io, Tycho Andersen <tycho@tycho.ws>,
	kernel list <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org,
	linux-arch <linux-arch@vger.kernel.org>,
	linux-kselftest@vger.kernel.org, dev@opencontainers.org,
	containers@lists.linux-foundation.org,
	Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution
Date: Sat, 29 Sep 2018 10:25:17 -0700
Message-ID: <F0E08B90-F10B-4897-913D-CA18E99A987D@amacapital.net> (raw)
In-Reply-To: <CAG48ez30WJhbsro2HOc_DR7V91M+hNFzBP5ogRMZaxbAORvqzg@mail.gmail.com>



> On Sep 29, 2018, at 9:35 AM, Jann Horn <jannh@google.com> wrote:
> 
> +cc linux-api; please keep them in CC for future versions of the patch
> 
>> On Sat, Sep 29, 2018 at 4:29 PM Aleksa Sarai <cyphar@cyphar.com> wrote:
>> The primary motivation for the need for this flag is container runtimes
>> which have to interact with malicious root filesystems in the host
>> namespaces. One of the first requirements for a container runtime to be
>> secure against a malicious rootfs is that they correctly scope symlinks
>> (that is, they should be scoped as though they are chroot(2)ed into the
>> container's rootfs) and ".."-style paths. The already-existing AT_XDEV
>> and AT_NO_PROCLINKS help defend against other potential attacks in a
>> malicious rootfs scenario.
> 
> So, I really like the concept for patch 1 of this series (but haven't
> read the code yet); but I dislike this patch because of its footgun
> potential.
> 

The code could do it differently: do the path walk and then, before accepting the result, walk back up and make sure the result is under the starting point.

This is *not* a full solution, though, since a walk above the root gas side effects on timing, various caches, and possibly network traffic, so it’s open to Spectre-like attacks in which a malicious container could use a runtime-initiated AT_THIS_ROOT to infer the existence of directories outside the container.

But what’s the container usecase?  Any sane container is based on pivot_root or similar, so the runtime can just do the walk in the container context. IOW I’m a bit confused as to the exact intended use of the whole series. Can you elaborate?

  reply index

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-29 10:34 [PATCH 0/3] namei: implement various scoping AT_* flags Aleksa Sarai
2018-09-29 10:34 ` [PATCH 1/3] namei: implement O_BENEATH-style " Aleksa Sarai
2018-09-29 14:48   ` Christian Brauner
2018-09-29 15:34     ` Aleksa Sarai
2018-09-30  4:38   ` Aleksa Sarai
2018-10-01 12:28   ` Jann Horn
2018-10-01 13:00     ` Christian Brauner
2018-10-01 16:04       ` Aleksa Sarai
2018-10-04 17:20         ` Christian Brauner
2018-09-29 13:15 ` [PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution Aleksa Sarai
2018-09-29 13:15   ` [PATCH 3/3] selftests: vfs: add AT_* path resolution tests Aleksa Sarai
2018-09-29 16:35   ` [PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution Jann Horn
2018-09-29 17:25     ` Andy Lutomirski [this message]
2018-10-01  9:46       ` Aleksa Sarai
2018-10-01  5:44     ` Aleksa Sarai
2018-10-01 10:13       ` Jann Horn
2018-10-01 16:18         ` Aleksa Sarai
2018-10-04 17:27           ` Christian Brauner
2018-10-01 10:42       ` Christian Brauner
2018-10-01 11:29         ` Jann Horn
2018-10-01 12:35           ` Christian Brauner
2018-10-01 13:55       ` Bruce Fields
2018-10-01 14:28       ` Andy Lutomirski
2018-10-02  7:32         ` Aleksa Sarai
2018-10-03 22:09           ` Andy Lutomirski
2018-10-06 20:56           ` Florian Weimer
2018-10-06 21:49             ` Christian Brauner
2018-10-01 14:00     ` Christian Brauner
2018-10-04 16:26     ` Aleksa Sarai
2018-10-04 17:31       ` Christian Brauner
2018-10-04 18:26       ` Jann Horn
2018-10-05 15:07         ` Aleksa Sarai
2018-10-05 15:55           ` Jann Horn
2018-10-06  2:10             ` Aleksa Sarai
2018-10-08 10:50               ` Jann Horn
2018-09-29 14:25 ` [PATCH 0/3] namei: implement various scoping AT_* flags Andy Lutomirski
2018-09-29 15:45   ` Aleksa Sarai
2018-09-29 16:34     ` Andy Lutomirski
2018-09-29 19:44       ` Matthew Wilcox
2018-09-29 14:38 ` Christian Brauner
2018-09-30  4:44   ` Aleksa Sarai
2018-09-30 13:54 ` Alban Crequy
2018-09-30 14:02   ` Christian Brauner
2018-09-30 19:45 ` Mickaël Salaün
2018-09-30 21:46   ` Jann Horn
2018-09-30 22:37     ` Mickaël Salaün
2018-10-01 20:14       ` James Morris
2018-10-01  4:08 ` Dave Chinner
2018-10-01  5:47   ` Aleksa Sarai
2018-10-01  6:14     ` Dave Chinner
2018-10-01 13:28 ` David Laight
2018-10-01 16:15   ` Aleksa Sarai
2018-10-03 13:21     ` David Laight

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