From: Andy Lutomirski <luto@amacapital.net>
To: Aleksa Sarai <cyphar@cyphar.com>
Cc: Jann Horn <jannh@google.com>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Jeff Layton <jlayton@kernel.org>,
"J. Bruce Fields" <bfields@fieldses.org>,
Al Viro <viro@zeniv.linux.org.uk>, Arnd Bergmann <arnd@arndb.de>,
Shuah Khan <shuah@kernel.org>,
David Howells <dhowells@redhat.com>,
Andrew Lutomirski <luto@kernel.org>,
Christian Brauner <christian@brauner.io>,
Tycho Andersen <tycho@tycho.ws>,
LKML <linux-kernel@vger.kernel.org>,
Linux FS Devel <linux-fsdevel@vger.kernel.org>,
linux-arch <linux-arch@vger.kernel.org>,
"open list:KERNEL SELFTEST FRAMEWORK"
<linux-kselftest@vger.kernel.org>,
dev@opencontainers.org,
Linux Containers <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: Wed, 3 Oct 2018 15:09:18 -0700 [thread overview]
Message-ID: <CALCETrXZ_Kg25gToCzzM=4g1Z3mQgou8RmF6OWoh3+vuPk3vUA@mail.gmail.com> (raw)
In-Reply-To: <20181002073220.7mzndna4tdnxdvdt@ryuk>
On Tue, Oct 2, 2018 at 12:32 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
>
> On 2018-10-01, Andy Lutomirski <luto@amacapital.net> wrote:
> > >>> Currently most container runtimes try to do this resolution in
> > >>> userspace[1], causing many potential race conditions. In addition, the
> > >>> "obvious" alternative (actually performing a {ch,pivot_}root(2))
> > >>> requires a fork+exec which is *very* costly if necessary for every
> > >>> filesystem operation involving a container.
> > >>
> > >> Wait. fork() I understand, but why exec? And actually, you don't need
> > >> a full fork() either, clone() lets you do this with some process parts
> > >> shared. And then you also shouldn't need to use SCM_RIGHTS, just keep
> > >> the file descriptor table shared. And why chroot()/pivot_root(),
> > >> wouldn't you want to use setns()?
> > >
> > > You're right about this -- for C runtimes. In Go we cannot do a raw
> > > clone() or fork() (if you do it manually with RawSyscall you'll end with
> > > broken runtime state). So you're forced to do fork+exec (which then
> > > means that you can't use CLONE_FILES and must use SCM_RIGHTS). Same goes
> > > for CLONE_VFORK.
> >
> > I must admit that I’m not very sympathetic to the argument that “Go’s
> > runtime model is incompatible with the simpler solution.”
>
> Multi-threaded programs have a similar issue (though with Go it's much
> worse). If you fork a multi-threaded C program then you can only safely
> use AS-Safe glibc functions (those that are safe within a signal
> handler). But if you're just doing three syscalls this shouldn't be as
> big of a problem as Go where you can't even do said syscalls.
>
> > Anyway, it occurs to me that the real problem is that setns() and
> > chroot() are both overkill for this use case.
>
> I agree. My diversion to Go was to explain why it was particularly bad
> for cri-o/rkt/runc/Docker/etc.
>
> > What’s needed is to start your walk from /proc/pid-in-container/root,
> > with two twists:
> >
> > 1. Do the walk as though rooted at a directory. This is basically just
> > your AT_THIS_ROOT, but the footgun is avoided because the dirfd you
> > use is from a foreign namespace, and, except for symlinks to absolute
> > paths, no amount of .. racing is going to escape the *namespace*.
>
> This is quite clever and I'll admit I hadn't thought of this. This
> definitely fixes the ".." issue, but as you've said it won't handle
> absolute symlinks (which means userspace has the same races that we
> currently have even if you assume that you have a container process
> already running -- CVE-2018-15664 is one of many examples of this).
>
> (AT_THIS_ROOT using /proc/$container/root would in principle fix all of
> the mentioned issues -- but as I said before I'd like to see whether
> hardening ".." would be enough to solve the escape problem.)
Hmm. Good point.
next prev parent reply other threads:[~2018-10-04 4:59 UTC|newest]
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
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 [this message]
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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