linux-kernel - Re: general protection fault in show

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Date:   Tue, 5 Dec 2017 11:58:07 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, security@...nel.org,
        Akinobu Mita <akinobu.mita@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        syzkaller-bugs@...glegroups.com,
        Vegard Nossum <vegard.nossum@...cle.com>
Subject: Re: general protection fault in show_timer

On Thu, Nov 30, 2017 at 4:57 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Thu, 30 Nov 2017, Alexey Dobriyan wrote:
>
>> [cc security@]
>> 100% oops with interrupts disabled by nobody
>> or kernel memory read
>> [nods]
>> you named the bug already
>>
>> "notify" directly comes from userspace struct sigevent::sigev_notify
>> without adult supervision.
>>
>> Reproducer is timer_create + read(/proc/self/timers)
>
> Bah. That's a really old one.
>
> Tentative fix below. That needs more though but looking at the existing
> check there is only one valid combo with SIGEV_THREAD_ID.
>
> Thanks,
>
>         tglx
>
> 8<----------------
> --- a/kernel/time/posix-timers.c
> +++ b/kernel/time/posix-timers.c
> @@ -434,6 +434,16 @@ static struct pid *good_sigevent(sigeven
>  {
>         struct task_struct *rtn = current->group_leader;
>
> +       switch (event->sigev_notify) {
> +       case SIGEV_NONE:
> +       case SIGEV_SIGNAL:
> +       case SIGEV_SIGNAL | SIGEV_THREAD_ID:
> +       case SIGEV_THREAD:
> +               break;
> +       default:
> +               return NULL;
> +       }
> +
>         if ((event->sigev_notify & SIGEV_THREAD_ID ) &&
>                 (!(rtn = find_task_by_vpid(event->sigev_notify_thread_id)) ||
>                  !same_thread_group(rtn, current) ||

FWIW, this looks correct to me.

-Kees

-- 
Kees Cook
Pixel Security