Re: [PATCH v8 1/1] ns: add binfmt_misc to the user namespace

[Jann Horn <jannh@google.com>]

On Mon, Dec 16, 2019 at 9:05 PM Laurent Vivier <laurent@vivier.eu> wrote:
> Le 16/12/2019 à 20:08, Jann Horn a écrit :
> > On Mon, Dec 16, 2019 at 10:12 AM Laurent Vivier <laurent@vivier.eu> wrote:
> >> This patch allows to have a different binfmt_misc configuration
> >> for each new user namespace. By default, the binfmt_misc configuration
> >> is the one of the previous level, but if the binfmt_misc filesystem is
> >> mounted in the new namespace a new empty binfmt instance is created and
> >> used in this namespace.
> >>
> >> For instance, using "unshare" we can start a chroot of another
> >> architecture and configure the binfmt_misc interpreter without being root
> >> to run the binaries in this chroot.
> >
> > How do you ensure that when userspace is no longer using the user
> > namespace and mount namespace, the entries and the binfmt_misc
> > superblock are deleted? As far as I can tell from looking at the code,
> > at the moment, if I create a user namespace+mount namespace, mount
> > binfmt_misc in there, register a file format and then let all
> > processes inside the namespaces exit, the binfmt_misc mount will be
> > kept alive by the simple_pin_fs() stuff, and the binfmt_misc entries
> > will also stay in memory.
> >
> > [...]
>
> Do you have any idea how I can fix this issue?

I think the easiest way (keeping in mind that we want to avoid having
to fiddle around with reference loops, where e.g. interpreter
executable files opened by binfmt_misc have references back to the
user namespace through ->f_cred) would be to add a new patch in front
of this one that changes the semantics such that when binfmt_misc is
unmounted, all the existing format registrations are deleted. That's
probably also nicer from the perspective of inspectability. It could
in theory break stuff, but I think that's probably somewhat unlikely.
Still, it'd be an API change, and therefore you should CC linux-api@
on such a change.

> >> @@ -718,7 +736,9 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
> >>         if (!inode)
> >>                 goto out2;
> >>
> >> -       err = simple_pin_fs(&bm_fs_type, &bm_mnt, &entry_count);
> >> +       ns = binfmt_ns(file_dentry(file)->d_sb->s_user_ns);
> >> +       err = simple_pin_fs(&bm_fs_type, &ns->bm_mnt,
> >> +                           &ns->entry_count);
> >
> > When you call simple_pin_fs() here, the user namespace of `current`
> > and the user namespace of the superblock are not necessarily related.
> > So simple_pin_fs() may end up taking a reference on the mountpoint for
> > a user namespace that has nothing to do with the namespace for which
> > an entry is being created.
>
> Do you have any idea how I can fix this issue?

If you fix the memory leak the way I suggested, this wouldn't be a
problem anymore either.

> >> +static void bm_free(struct fs_context *fc)
> >> +{
> >> +       if (fc->s_fs_info)
> >> +               put_user_ns(fc->s_fs_info);
> >> +}
> >
> > Silly question: Why the "if"? Can you ever reach this with fc->s_fs_info==NULL?
>
> So I understand the if is unnecessary and I will remove it.

Your code was actually exactly right, I didn't understand how
fc->s_fs_info works.

> >>  static int bm_get_tree(struct fs_context *fc)
> >>  {
> >> -       return get_tree_single(fc, bm_fill_super);
> >> +       return get_tree_keyed(fc, bm_fill_super, get_user_ns(fc->user_ns));
> >
> > get_user_ns() increments the refcount of the namespace, but in the
> > case where a binfmt_misc mount already exists, that refcount is never
> > dropped, right? That would be a security bug, since an attacker could
> > overflow the refcount of the user namespace and then trigger a UAF.
> > (And the refcount hardening won't catch it because user namespaces
> > still use raw atomics instead of refcount_t.)
>
> Do you have any idea how I can fix this issue?

Ah, this was actually fine. I missed that get_tree_keyed() stashes
that pointer in fc->s_fs_info.

> >> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> >
> > Nit: Isn't this kind of check normally written as "#ifdef"?
> >
>
> What is the difference?

As explained in Documentation/process/coding-style.rst and the
relevant header, IS_ENABLED() is for inline use in C expressions.