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Date: Thu, 30 Nov 2017 13:57:55 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Alexey Dobriyan <adobriyan@gmail.com>
cc: Dmitry Vyukov <dvyukov@google.com>, security@kernel.org,
        Akinobu Mita <akinobu.mita@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
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        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        syzkaller-bugs@googlegroups.com,
        Vegard Nossum <vegard.nossum@oracle.com>
Subject: Re: general protection fault in show_timer
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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) ||