Re: [PATCH] seccomp: add ptrace commands for suspend/resume

On Tue, Jun 02, 2015 at 10:02:10PM +0300, Pavel Emelyanov wrote:

quoted

quoted

+int suspend_seccomp(struct task_struct *task)
+{
+	int ret = -EACCES;
+
+	spin_lock_irq(&task->sighand->siglock);
+
+	if (!capable(CAP_SYS_ADMIN))
+		goto out;

I am puzzled ;) Why do we need ->siglock? And even if we need it, why
we can't check CAP_SYS_ADMIN lockless?

And I am not sure I understand why do we need the additional security
check, but I leave this to you and Andy.

If you have the rights to trace this task, then you can do anything
the tracee could do without the filtering.

I think _this_ check is required, otherwise the seccomp-ed task (in
filtered mode) fork-s a child, then this child ptrace-attach to parent
(allowed) then suspend its seccomd. And -- we have unpriviledged process 
de-seccomped.

If you can ptrace(), you can already escape from seccomp. See this
section in man 2 seccomp, in the SECCOMP_RET_TRACE section:

              The seccomp check will not be run again after the tracer is
              notified.  (This means that seccomp-based sandboxes must not
              allow use of ptrace(2)—even of other sandboxed processes—
              without extreme care; ptracers can use this mechanism to
              escape from the seccomp sandbox.)

(But I think there have been discussions about changing that behavior in
the future?)