netdev - Re: [PATCH v2 4/5] seccomp: add a way to access filters via bpf fds

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Date:	Fri, 11 Sep 2015 09:20:55 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Tycho Andersen <tycho.andersen@...onical.com>
Cc:	Pavel Emelyanov <xemul@...allels.com>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Alexei Starovoitov <ast@...nel.org>,
	Will Drewry <wad@...omium.org>,
	Network Development <netdev@...r.kernel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 4/5] seccomp: add a way to access filters via bpf fds

On Sep 10, 2015 5:22 PM, "Tycho Andersen" <tycho.andersen@...onical.com> wrote:
>
> This patch adds a way for a process that is "real root" to access the
> seccomp filters of another process. The process first does a
> PTRACE_SECCOMP_GET_FILTER_FD to get an fd with that process' seccomp filter
> attached, and then iterates on this with PTRACE_SECCOMP_NEXT_FILTER using
> bpf(BPF_PROG_DUMP) to dump the actual program at each step.
>

> +
> +       fd = bpf_new_fd(filter->prog, O_RDONLY);
> +       if (fd > 0)
> +               atomic_inc(&filter->prog->aux->refcnt);

Why isn't this folded into bpf_new_fd?

> +
> +       return fd;
> +}
> +
> +long seccomp_next_filter(struct task_struct *child, u32 fd)
> +{
> +       struct seccomp_filter *cur;
> +       struct bpf_prog *prog;
> +       long ret = -ESRCH;
> +
> +       if (!capable(CAP_SYS_ADMIN))
> +               return -EACCES;
> +
> +       if (child->seccomp.mode != SECCOMP_MODE_FILTER)
> +               return -EINVAL;
> +
> +       prog = bpf_prog_get(fd);
> +       if (IS_ERR(prog)) {
> +               ret = PTR_ERR(prog);
> +               goto out;
> +       }
> +
> +       for (cur = child->seccomp.filter; cur; cur = cur->prev) {
> +               if (cur->prog == prog) {
> +                       if (!cur->prev)
> +                               ret = -ENOENT;
> +                       else
> +                               ret = bpf_prog_set(fd, cur->prev->prog);

This lets you take an fd pointing to one prog and point it elsewhere.
I'm not sure that's a good idea.

--Andy
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