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

On Thu, Sep 27, 2018 at 07:35:06PM +0200, Jann Horn wrote:

On Thu, Sep 27, 2018 at 5:11 PM Tycho Andersen [off-list ref] wrote:

quoted

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.

[...]

quoted

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`.

Yes, good idea. Thanks!

Tycho