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From: Aleksa Sarai <cyphar@cyphar.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Jann Horn <jannh@google.com>, Andy Lutomirski <luto@kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Jeff Layton <jlayton@kernel.org>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	Arnd Bergmann <arnd@arndb.de>,
	David Howells <dhowells@redhat.com>,
	Eric Biederman <ebiederm@xmission.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Christian Brauner <christian@brauner.io>,
	Tycho Andersen <tycho@tycho.ws>,
	David Drysdale <drysdale@google.com>,
	Chanho Min <chanho.min@lge.com>, Oleg Nesterov <oleg@redhat.com>,
	Aleksa Sarai <asarai@suse.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Linux Containers <containers@lists.linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [PATCH v6 5/6] binfmt_*: scope path resolution of interpreters
Date: Sun, 12 May 2019 03:26:04 +1000
Message-ID: <20190511172604.znr7wa3iarlgzor4@yavin> (raw)
In-Reply-To: <C60DC580-854D-478D-AF23-5F29FB7C3E50@amacapital.net>


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On 2019-05-11, Andy Lutomirski <luto@amacapital.net> wrote:
> >> I've lost track of the context here, but it seems to me that
> >> mitigating attacks involving accidental following of /proc links
> >> shouldn't depend on dumpability.  What's the actual problem this is
> >> trying to solve again?
> > 
> > The one actual security problem that I've seen related to this is
> > CVE-2019-5736. There is a write-up of it at
> > <https://blog.dragonsector.pl/2019/02/cve-2019-5736-escape-from-docker-and.html>
> > under "Successful approach", but it goes more or less as follows:
> > 
> > A container is running that doesn't use user namespaces (because for
> > some reason I don't understand, apparently some people still do that).
> > An evil process is running inside the container with UID 0 (as in,
> > GLOBAL_ROOT_UID); so if the evil process inside the container was able
> > to reach root-owned files on the host filesystem, it could write into
> > them.
> > 
> > The container engine wants to spawn a new process inside the container.
> > It forks off a child that joins the container's namespaces (including
> > PID and mount namespaces), and then the child calls execve() on some
> > path in the container.
> 
> I think that, at this point, the task should be considered owned by
> the container.  Maybe we should have a better API than execve() to
> execute a program in a safer way, but fiddling with dumpability seems
> like a band-aid.  In fact, the process is arguably pwned even *before*
> execve.

Yeah, execve is just the vector (though in this case it's done in order
to clear mm->dumpable). An earlier CVE (CVE-2016-9962) was very similar
but was attacking a dirfd that runc had open into the container (LXC had
a very similar bug too) -- setting !mm->dumpable was one of the
workarounds we had for this.

> A better “spawn” API should fix this.  In the mean time, I think it
> should be assumed that, if you join a container’s namespaces, you are
> at its mercy.

This is generally how we treat containers as runtime authors, but it's
not a trivial thing to get right. In many cases the kernel APIs are
working against you -- Christian and myself have written a fair few
patches to fix holes in the kernel APIs so we can avoid these kinds of
assumptions.

But yes, one of the most risky parts of a container runtime is when
you're attaching to a running container because all of the helpful
introspection APIs in /proc/ suddenly become a security nightmare. A
better "spawn a process in these namespaces" API might help improve the
situation (or at least, I hope it would).

> > - You can use /proc/*/exe to get a writable fd.
> 
> This is IMO the real bug.

I will try to send an RFC of the patchset I have for this next week or
so. Funnily enough, currently /proc/*/exe has the write bit set in its
"mode" (my series fixes this).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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  parent reply index

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-06 16:54 [PATCH v6 0/6] namei: resolveat(2) path resolution restriction API Aleksa Sarai
2019-05-06 16:54 ` [PATCH v6 1/6] namei: split out nd->dfd handling to dirfd_path_init Aleksa Sarai
2019-05-06 16:54 ` [PATCH v6 2/6] namei: O_BENEATH-style path resolution flags Aleksa Sarai
2019-05-06 16:54 ` [PATCH v6 3/6] namei: LOOKUP_IN_ROOT: chroot-like path resolution Aleksa Sarai
2019-05-06 16:54 ` [PATCH v6 4/6] namei: aggressively check for nd->root escape on ".." resolution Aleksa Sarai
2019-05-06 16:54 ` [PATCH v6 5/6] binfmt_*: scope path resolution of interpreters Aleksa Sarai
2019-05-06 18:37   ` Jann Horn
2019-05-06 19:17     ` Aleksa Sarai
2019-05-06 23:41       ` Andy Lutomirski
2019-05-08  0:54       ` Aleksa Sarai
2019-05-10 20:41       ` Jann Horn
2019-05-10 21:20         ` Andy Lutomirski
2019-05-10 22:55           ` Jann Horn
2019-05-10 23:36             ` Christian Brauner
2019-05-11 15:49               ` Aleksa Sarai
2019-05-11 17:00             ` Andy Lutomirski
2019-05-11 17:21               ` Linus Torvalds
2019-05-11 17:26                 ` Linus Torvalds
2019-05-11 17:31                   ` Aleksa Sarai
2019-05-11 17:43                     ` Linus Torvalds
2019-05-11 17:48                       ` Christian Brauner
2019-05-11 18:00                       ` Aleksa Sarai
2019-05-11 22:39                 ` Andy Lutomirski
     [not found]                   ` <CAHk-=wg3+3GfHsHdB4o78jNiPh_5ShrzxBuTN-Y8EZfiFMhCvw@mail.gmail.com>
2019-05-12 10:19                     ` Christian Brauner
     [not found]                     ` <9CD2B97D-A6BD-43BE-9040-B410D996A195@amacapital.net>
2019-05-12 10:44                       ` Linus Torvalds
2019-05-12 13:35                         ` Aleksa Sarai
2019-05-12 13:38                           ` Aleksa Sarai
2019-05-12 14:34                           ` Andy Lutomirski
2019-05-11 17:26               ` Aleksa Sarai [this message]
2019-05-08  0:38     ` Eric W. Biederman
2019-05-10 20:10       ` Jann Horn
2019-05-06 16:54 ` [PATCH v6 6/6] namei: resolveat(2) syscall Aleksa Sarai

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