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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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