LKML Archive on lore.kernel.org
 help / color / Atom feed
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
Message-ID: <20190506191735.nmzf7kwfh7b6e2tf@yavin> (raw)
In-Reply-To: <CAG48ez0-CiODf6UBHWTaog97prx=VAd3HgHvEjdGNz344m1xKw@mail.gmail.com>


[-- Attachment #1: Type: text/plain, Size: 4411 bytes --]

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/>

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]

  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 [this message]
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
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190506191735.nmzf7kwfh7b6e2tf@yavin \
    --to=cyphar@cyphar.com \
    --cc=akpm@linux-foundation.org \
    --cc=arnd@arndb.de \
    --cc=asarai@suse.de \
    --cc=ast@kernel.org \
    --cc=bfields@fieldses.org \
    --cc=chanho.min@lge.com \
    --cc=christian@brauner.io \
    --cc=containers@lists.linux-foundation.org \
    --cc=dhowells@redhat.com \
    --cc=drysdale@google.com \
    --cc=ebiederm@xmission.com \
    --cc=jannh@google.com \
    --cc=jlayton@kernel.org \
    --cc=keescook@chromium.org \
    --cc=linux-api@vger.kernel.org \
    --cc=linux-arch@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=luto@kernel.org \
    --cc=oleg@redhat.com \
    --cc=torvalds@linux-foundation.org \
    --cc=tycho@tycho.ws \
    --cc=viro@zeniv.linux.org.uk \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git