Re: [PATCH 2/2] ARM: futex: make futex_detect_cmpxchg more reliable

[Ard Biesheuvel]

On Mon, 11 Mar 2019 at 17:30, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Mon, Mar 11, 2019 at 3:36 PM Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
> > On Mon, 11 Mar 2019 at 15:34, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > > On Fri, Mar 8, 2019 at 12:56 PM Ard Biesheuvel
> > > <ard.biesheuvel@xxxxxxxxxx> wrote:
> > > > On Fri, 8 Mar 2019 at 11:58, Russell King - ARM Linux admin <linux@xxxxxxxxxxxxxxx> wrote:
> > > > > On Fri, Mar 08, 2019 at 11:45:21AM +0100, Ard Biesheuvel wrote:
> > >
> > > My first attempt (before finding the original patch from Mikael Pettersson)
> > > was to change the probe to pass '1' as the value instead of '0', that
> > > worked fine.
> > >
> >
> > Which probe is that?
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index c3b73b0311bc..19615ad3c4f7 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -3864,7 +3864,7 @@ static void __init futex_detect_cmpxchg(void)
>          * implementation, the non-functional ones will return
>          * -ENOSYS.
>          */
> -       if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
> +       if (cmpxchg_futex_value_locked(&curval, NULL, 1, 1) == -EFAULT)
>                 futex_cmpxchg_enabled = 1;
>  #endif
>  }
>

Ah ok.

That explains a lot.

Can't we just return -EFAULT if uaddr is NULL? Or does that defeat this check?

Note that PA-RISC has

        /* futex.c wants to do a cmpxchg_inatomic on kernel NULL, which is
         * our gateway page, and causes no end of trouble...
         */
        if (uaccess_kernel() && !uaddr)
                return -EFAULT;