On Fri, Sep 28, 2018 at 10:56 PM Tycho Andersen <[email protected]> wrote: > > On Fri, Sep 28, 2018 at 10:33:34PM +0200, Jann Horn wrote: > > On Fri, Sep 28, 2018 at 5:47 PM Tycho Andersen <[email protected]> wrote: > > > As Jann pointed out, there is a race between SECCOMP_FILTER_FLAG_TSYNC and > > > the ptrace code that can inspect a filter of another process. Let's > > > introduce read locking into the two ptrace accesses so that we don't race. > > > > Hmm. Is that true? The ptrace code uses get_nth_filter(), which holds > > the siglock while grabbing the seccomp filter and bumping its > > refcount. And TSYNC happens from seccomp_set_mode_filter(), which > > takes the siglock. So this looks okay to me? > > Oh, yes, you're right. So I guess we should just change the comment to > say we're using siglock to represent the read lock.
Hmm... actually, looking at this closer, I think you only need the siglock for writing. As far as I can tell, any read (no matter if current or non-current) can just use READ_ONCE(), because once a seccomp filter is in a task's seccomp filter chain, it can't be freed until the task reaches free_task() and calls put_seccomp_filter() from there. And if the task whose seccomp filter you're trying to read can reach free_task(), you have bigger problems.
