Re: [PATCH v6 5/6] binfmt_*: scope path resolution of interpreters

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: Tue, 7 May 2019 05:17:35 +1000	[thread overview]
Message-ID: <20190506191735.nmzf7kwfh7b6e2tf@yavin> (raw)
In-Reply-To: <CAG48ez0-CiODf6UBHWTaog97prx=VAd3HgHvEjdGNz344m1xKw@mail.gmail.com>

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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.

> In my opinion, CVE-2019-5736 points out two different problems:
>
> The big problem: The __ptrace_may_access() logic has a special-case
> short-circuit for "introspection" that you can't opt out of; this
> makes it possible to open things in procfs that are related to the
> current process even if the credentials of the process wouldn't permit
> accessing another process like it. I think the proper fix to deal with
> this would be to add a prctl() flag for "set whether introspection is
> allowed for this process", and if userspace has manually un-set that
> flag, any introspection special-case logic would be skipped.

We could do PR_SET_DUMPABLE=3 for this, I guess?

> An additional problem: /proc/*/exe can be used to open a file for
> writing; I think it may have been Andy Lutomirski who pointed out some
> time ago that it would be nice if you couldn't use /proc/*/fd/* to
> re-open files with more privileges, which is sort of the same thing.

This is something I'm currently working on a series for, which would
boil down to some restrictions on how re-opening of file descriptors
works through procfs.

However, execveat() of a procfs magiclink is a bit hard to block --
there is no way for userspace to to represent a file being "open for
execute" so they are all "open for execute" by default and blocking it
outright seems a bit extreme (though I actually hope to eventually add
the ability to mark an O_PATH as "open for X" to resolveat(2) -- hence
why I've reserved some bits).

(Thinking more about it, there is an argument that I should include the
above patch into this series so that we can block re-opening of fds
opened through resolveat(2) without explicit flags from the outset.)

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

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