Messages in this thread | | | Date | Fri, 28 Sep 2018 16:02:43 -0600 | From | Tycho Andersen <> | Subject | Re: [PATCH 3/3] seccomp: introduce read protection for struct seccomp |
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On Fri, Sep 28, 2018 at 11:54:22PM +0200, Jann Horn wrote: > On Fri, Sep 28, 2018 at 11:36 PM Tycho Andersen <tycho@tycho.ws> wrote: > > On Fri, Sep 28, 2018 at 11:10:48PM +0200, Jann Horn wrote: > > > On Fri, Sep 28, 2018 at 10:56 PM Tycho Andersen <tycho@tycho.ws> 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 <tycho@tycho.ws> 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. > > > > Ok; looks like get_nth_filter() took the siglock anyway. Since we get > > the filters in these two functions in get_nth_filter(), I think it's > > enough just to just, > > > > diff --git a/kernel/seccomp.c b/kernel/seccomp.c > > index f65d47650ac1..79d833ed4c34 100644 > > --- a/kernel/seccomp.c > > +++ b/kernel/seccomp.c > > @@ -1001,7 +1001,7 @@ static struct seccomp_filter *get_nth_filter(struct task_struct *task, > > return ERR_PTR(-EINVAL); > > } > > > > - orig = task->seccomp.filter; > > + orig = READ_ONCE(task->seccomp.filter); > > __get_seccomp_filter(orig); > > spin_unlock_irq(&task->sighand->siglock); > > Huh? Now you're holding the siglock *and* you're using READ_ONCE()? > I'm not sure what you're trying to accomplish here.
Yes, let's just drop this patch all together.
Tycho
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