Re: [PATCH v4 4/4] seccomp: add support for passing fds via USER_NOTIF

[Jann Horn <jannh@google.com>]

On Fri, Jun 22, 2018 at 12:05 AM Tycho Andersen <tycho@tycho.ws> wrote:
>
> The idea here is that the userspace handler should be able to pass an fd
> back to the trapped task, for example so it can be returned from socket().
>
> I've proposed one API here, but I'm open to other options. In particular,
> this only lets you return an fd from a syscall, which may not be enough in
> all cases. For example, if an fd is written to an output parameter instead
> of returned, the current API can't handle this. Another case is that
> netlink takes as input fds sometimes (IFLA_NET_NS_FD, e.g.). If netlink
> ever decides to install an fd and output it, we wouldn't be able to handle
> this either.
>
> Still, the vast majority of interesting cases are covered by this API, so
> perhaps it is Enough.
>
> I've left it as a separate commit for two reasons:
>   * It illustrates the way in which we would grow struct seccomp_notif and
>     struct seccomp_notif_resp without using netlink
>   * It shows just how little code is needed to accomplish this :)
>
[...]
> @@ -1669,10 +1706,20 @@ static ssize_t seccomp_notify_write(struct file *file, const char __user *buf,
>                 goto out;
>         }
>
> +       if (resp.return_fd) {
> +               knotif->flags = resp.fd_flags;
> +               knotif->file = fget(resp.fd);
> +               if (!knotif->file) {
> +                       ret = -EBADF;
> +                       goto out;
> +               }
> +       }
> +

I think this is a security bug. Imagine the following scenario:

 - attacker creates processes A and B
 - process A installs a seccomp filter and sends the notification fd
to process B
 - process A starts a syscall for which the filter returns
SECCOMP_RET_USER_NOTIF
 - process B reads the notification from the notification fd
 - process B uses dup2() to copy the notification fd to file
descriptor 1 (stdout)
 - process B executes a setuid root binary
 - the setuid root binary opens some privileged file descriptor
(something like open("/etc/shadow", O_RDWR))
 - the setuid root binary tries to write some attacker-controlled data to stdout
 - seccomp_notify_write() interprets the start of the written data as
a struct seccomp_notif_resp
 - seccomp_notify_write() grabs the privileged file descriptor and
installs a copy in process A
 - process A now has access to the privileged file (e.g. /etc/shadow)

It isn't clear whether it would actually be exploitable - you'd need a
setuid binary that performs the right actions - but it's still bad.

Unless I'm missing something, can you please turn the ->read and
->write handlers into an ->unlocked_ioctl handler? Something like
this:

struct seccomp_user_notif_args {
        u64 buf;
        u64 size;
};

static long unlocked_ioctl(struct file *file, unsigned int cmd,
unsigned long arg)
{
        struct seccomp_user_notif_args args;
        struct seccomp_user_notif_args __user *uargs;

        if (cmd != SECCOMP_USER_NOTIF_READ && cmd != SECCOMP_USER_NOTIF_WRITE)
                return -EINVAL;

        if (copy_from_user(&args, uargs, sizeof(args)))
                return -EFAULT;

        switch (cmd) {
        case SECCOMP_USER_NOTIF_READ:
                return seccomp_notify_read(file, (char __user
*)args.buf, (size_t)args.size);
        case SECCOMP_USER_NOTIF_WRITE:
                return seccomp_notify_write(file, (char __user
*)args.buf, (size_t)args.size);
        default:
                return -EINVAL;
        }
}