On 2019-05-06, Jann Horn [off-list ref] wrote:

On Mon, May 6, 2019 at 6:56 PM Aleksa Sarai [off-list ref] wrote:

quoted

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.

quoted

@@ -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);

[...]

quoted

+#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/>