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From: Aleksa Sarai <cyphar@cyphar.com>
To: Jann Horn <jannh@google.com>
Cc: 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>,
	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: Wed, 8 May 2019 10:54:56 +1000
Message-ID: <20190508005456.th5pkxljz536cq6w@yavin> (raw)
In-Reply-To: <20190506191735.nmzf7kwfh7b6e2tf@yavin>


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On 2019-05-07, Aleksa Sarai <cyphar@cyphar.com> wrote:
> On 2019-05-06, Jann Horn <jannh@google.com> wrote:
> > On Mon, May 6, 2019 at 6:56 PM Aleksa Sarai <cyphar@cyphar.com> wrote:
> > > The need to be able to scope path resolution of interpreters became
> > > clear with one of the possible vectors used in CVE-2019-5736 (which
> > > most major container runtimes were vulnerable to).
> > >
> > > Naively, it might seem that openat(2) -- which supports path scoping --
> > > can be combined with execveat(AT_EMPTY_PATH) to trivially scope the
> > > binary being executed. Unfortunately, a "bad binary" (usually a symlink)
> > > could be written as a #!-style script with the symlink target as the
> > > interpreter -- which would be completely missed by just scoping the
> > > openat(2). An example of this being exploitable is CVE-2019-5736.
> > >
> > > In order to get around this, we need to pass down to each binfmt_*
> > > implementation the scoping flags requested in execveat(2). In order to
> > > maintain backwards-compatibility we only pass the scoping AT_* flags.
> > >
> > > To avoid breaking userspace (in the exceptionally rare cases where you
> > > have #!-scripts with a relative path being execveat(2)-ed with dfd !=
> > > AT_FDCWD), we only pass dfd down to binfmt_* if any of our new flags are
> > > set in execveat(2).
> > 
> > This seems extremely dangerous. I like the overall series, but not this patch.
> > 
> > > @@ -1762,6 +1774,12 @@ static int __do_execve_file(int fd, struct filename *filename,
> > >
> > >         sched_exec();
> > >
> > > +       bprm->flags = flags & (AT_XDEV | AT_NO_MAGICLINKS | AT_NO_SYMLINKS |
> > > +                              AT_THIS_ROOT);
> > [...]
> > > +#define AT_THIS_ROOT           0x100000 /* - Scope ".." resolution to dirfd (like chroot(2)). */
> > 
> > So now what happens if there is a setuid root ELF binary with program
> > interpreter "/lib64/ld-linux-x86-64.so.2" (like /bin/su), and an
> > unprivileged user runs it with execveat(..., AT_THIS_ROOT)? Is that
> > going to let the unprivileged user decide which interpreter the
> > setuid-root process should use? From a high-level perspective, opening
> > the interpreter should be controlled by the program that is being
> > loaded, not by the program that invoked it.
> 
> I went a bit nuts with openat_exec(), and I did end up adding it to the
> ELF interpreter lookup (and you're completely right that this is a bad
> idea -- I will drop it from this patch if it's included in the next
> series).
> 
> The proposed solutions you give below are much nicer than this patch so
> I can drop it and work on fixing those issues separately.

Another possible solution would be to only allow (for instance)
AT_NO_MAGICLINKS for execveat(2). That way you cannot scope the
resolution but you can block the most concerning cases -- those
involving /proc-related access.

I've posted a v7 with this patch dropped (because we can always add AT_*
flags later in time), but I think having at least NO_MAGICLINKS would be
useful.

-- 
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 [this message]
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
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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