Re: [GIT PULL] ucounts: Count rlimits in each user namespace

[Alexey Gladkov <legion@kernel.org>]

On Tue, Jun 29, 2021 at 01:33:39PM -0700, Linus Torvalds wrote:
> On Tue, Jun 29, 2021 at 1:20 PM Alexey Gladkov <legion@kernel.org> wrote:
> >
> > Waaaait. task_ucounts() is a different thing. This function only gets a
> > field from the task structure without any reference counting. But the
> > get_ucounts() is more like get_user_ns() or get_uid(), but does not ignore
> > counter overflow.
> 
> Alexey, that code cannot be right.
> 
> Look here:
> 
>         rcu_read_lock();
>         ucounts = task_ucounts(t);
>         sigpending = inc_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING, 1);
>         if (sigpending == 1)
>                 ucounts = get_ucounts(ucounts);
>         rcu_read_unlock();
> 
> so now we've done that inc_rlimit_ucounts() unconditionally on that
> task_ucounts() thing.
> 
> And then if the allocation fails (or the limit is hit) the code does
> 
>         if (ucounts && dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING, 1))
>                 put_ucounts(ucounts);
> 
> ie now it does the dec_rlimit_ucounts _conditionally_.
> 
> See what I'm complaining about? This is not logical, AND IT CANNOT
> POSSIBLY BE RIGHT.
> 
> My argument is that
> 
>  (a) the dec_rlimit_ucounts() has to pair up with
> inc_rlimit_ucounts(), or you're leaking counts
> 
>  (b) get_ucounts() has to pair up with put_ucounts().
> 
> Note that (a) has to be REGARDLESS of whether get_ucounts() was
> successful or not.
> 
> > Earlier I tried to use refcount_t which never returns errors [1]. We
> > talked and you said that ignoring counter overflow errors is bad
> > design for this case.
> 
> You can't ignore counter overflow errors, no. But that's exactly what
> that code is doing.
> 
> If get_ucount() fails due to overflow, you don't return an error. You
> just miscount the end result!
> 
> So yeah, its' "testing" the overflow condition, but that's not an
> argument, when it then DOES EXPLICITLY THE WRONG THING.
> 
> At that point, the test is actively harmful and wrong. See?

Yes. Please, give me some time to fix it.

-- 
Rgrds, legion