Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace

Jann Horn
2018-09-27T17:35:37Z

On Thu, Sep 27, 2018 at 5:11 PM Tycho Andersen <tycho@tycho.ws> wrote:
>
> As an alternative to SECCOMP_FILTER_FLAG_GET_LISTENER, perhaps a ptrace()
> version which can acquire filters is useful. There are at least two reasons
> this is preferable, even though it uses ptrace:
>
> 1. You can control tasks that aren't cooperating with you
> 2. You can control tasks whose filters block sendmsg() and socket(); if the
> task installs a filter which blocks these calls, there's no way with
> SECCOMP_FILTER_FLAG_GET_LISTENER to get the fd out to the privileged task.
[...]
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 21fec73d45d4..289960ac181b 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -1096,6 +1096,10 @@ int ptrace_request(struct task_struct *child, long request,
> ret = seccomp_get_metadata(child, addr, datavp);
> break;
>
> + case PTRACE_SECCOMP_NEW_LISTENER:
> + ret = seccomp_new_listener(child, addr);
> + break;

Actually, could you amend this to also ensure that `data == 0` and
return -EINVAL otherwise? Then if we want to abuse `data` for passing
flags in the future, we don't have to worry about what happens if
someone passes in garbage as `data`.