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Date:   Thu, 2 Nov 2017 18:01:38 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot 
        <bot+c9f0eb0d2a5576ece331a767528e6b52b4ff1815@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arvind Yadav <arvind.yadav.cs@...il.com>,
        Mark Brown <broonie@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        jamie.iles@...cle.com, LKML <linux-kernel@...r.kernel.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        mchehab@...nel.org, Ingo Molnar <mingo@...nel.org>,
        mpe@...erman.id.au, syzkaller-bugs@...glegroups.com,
        Al Viro <viro@...iv.linux.org.uk>, Kyle Huey <me@...ehuey.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: WARNING in task_participate_group_stop

On 11/01, Dmitry Vyukov wrote:
>
> On Tue, Oct 31, 2017 at 7:34 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> > Hmm. I do not see reproducer in this email...
>
> Ah, sorry. You can see full thread with attachments here:
> https://groups.google.com/forum/#!topic/syzkaller-bugs/EUmYZU4m5gU

Heh. I can't say I enjoyed reading the reproducer ;)

> >> > WARNING: CPU: 0 PID: 1 at kernel/signal.c:340
> >> > task_participate_group_stop+0x1ce/0x230 kernel/signal.c:340
> >> > Kernel panic - not syncing: panic_on_warn set ...
> >> >
> >> > CPU: 0 PID: 1 Comm: init Not tainted 4.13.0-mm1+ #5
> >
> > So this is init process with SIGNAL_UNKILLABLE flag set. And I hope it has
> > the pending SIGKILL, otherwise there is something else.

>From repro.c

	line 111    r[8] = syscall(__NR_ptrace, 0x10ul, r[7]);

this is PTRACE_ATTACH

	line 115        syscall(__NR_ptrace, 0x4200ul, r[7], 0x40000012ul, 0x100012ul);
	
this is PTRACE_SETOPTIONS and "data" includes PTRACE_O_EXITKILL.

r[7] is initialized at

	line 110      r[7] = *(uint32_t*)0x20f9cffc;

so if it is eq to 1 then it can attach to init and in this case the problem
can be explained by the wrong SIGNAL_UNKILLABLE/SIGKILL logic.

But how *(uint32_t*)0x20f9cffc can be 1 ?

	line 108    r[6] = syscall(__NR_fcntl, r[1], 0x10ul, 0x20f9cff8ul);
	
this is F_GETOWN_EX, addr = 0x20f9cff8 == 0x20f9cffc + 4, so if fcntl()
actually succeeds then r[7] == f_owner_ex->pid.

It _can_ be 1, but the reproducer doesn't work for me. If you can reproduce,
could you try the patch below?

Oleg.


diff --git a/kernel/signal.c b/kernel/signal.c
index 800a18f..7e15b56 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -78,7 +78,7 @@ static int sig_task_ignored(struct task_struct *t, int sig, bool force)
 	handler = sig_handler(t, sig);
 
 	if (unlikely(t->signal->flags & SIGNAL_UNKILLABLE) &&
-			handler == SIG_DFL && !force)
+	    handler == SIG_DFL && !(force && sig_kernel_only(sig)))
 		return 1;
 
 	return sig_handler_ignored(handler, sig);
@@ -94,13 +94,15 @@ static int sig_ignored(struct task_struct *t, int sig, bool force)
 	if (sigismember(&t->blocked, sig) || sigismember(&t->real_blocked, sig))
 		return 0;
 
-	if (!sig_task_ignored(t, sig, force))
-		return 0;
-
 	/*
-	 * Tracers may want to know about even ignored signals.
+	 * Tracers may want to know about even ignored signal unless it
+	 * is SIGKILL which can't be reported anyway but can be ignored
+	 * by SIGNAL_UNKILLABLE task.
 	 */
-	return !t->ptrace;
+	if (t->ptrace && sig != SIGKILL)
+		return 0;
+
+	return sig_task_ignored(t, sig, force);
 }
 
 /*
@@ -929,9 +931,9 @@ static void complete_signal(int sig, struct task_struct *p, int group)
 	 * then start taking the whole group down immediately.
 	 */
 	if (sig_fatal(p, sig) &&
-	    !(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) &&
+	    !(signal->flags & SIGNAL_GROUP_EXIT) &&
 	    !sigismember(&t->real_blocked, sig) &&
-	    (sig == SIGKILL || !t->ptrace)) {
+	    (sig == SIGKILL || !p->ptrace)) {
 		/*
 		 * This signal will be fatal to the whole group.
 		 */

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