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