Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756240AbbKDSXD (ORCPT ); Wed, 4 Nov 2015 13:23:03 -0500 Received: from mx1.redhat.com ([209.132.183.28]:45494 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756158AbbKDSXB (ORCPT ); Wed, 4 Nov 2015 13:23:01 -0500 Date: Wed, 4 Nov 2015 20:19:12 +0100 From: Oleg Nesterov To: Andrew Morton , Dmitry Vyukov Cc: Roland McGrath , amanieu@gmail.com, pmoore@redhat.com, Ingo Molnar , vdavydov@parallels.com, qiaowei.ren@intel.com, dave@stgolabs.net, palmer@dabbelt.com, LKML , syzkaller , Kostya Serebryany , Alexander Potapenko , Sasha Levin Subject: [PATCH 1/1] signal: kill the obsolete SIGNAL_UNKILLABLE check in complete_signal() Message-ID: <20151104191912.GB20578@redhat.com> References: <20151102151333.GA17152@redhat.com> <20151102163654.GA18804@redhat.com> <20151102180624.GA24361@redhat.com> <20151104191804.GA20578@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151104191804.GA20578@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2701 Lines: 84 complete_signal() checks SIGNAL_UNKILLABLE before it starts to destroy the thread group, today this is unnecessary and even not 100% correct. After the commit f008faff0e27 ("signals: protect init from unwanted signals more") we rely on sig_task_ignored(), complete_signal(SIGKILL) can only see a SIGNAL_UNKILLABLE task if we actually want to kill it. And note that after the commit b3bfa0cba867 ("signals: protect cinit from blocked fatal signals") we do not drop SIGKILL dequeued by /sbin/init. And it does not look right. fatal_signal_pending() should always imply that the whole thread group (except ->group_exit_task if it is not NULL) is killed, this check breaks the rule. This explains WARN_ON(!JOBCTL_STOP_PENDING) in task_participate_group_stop() triggered by the test-case from Dmitry: int main() { int pid = 1; ptrace(PTRACE_ATTACH, pid, 0, 0); ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_EXITKILL); sleep(1); return 0; } do_signal_stop()->signal_group_exit() returns false because SIGNAL_GROUP_EXIT is not set, but task_set_jobctl_pending() checks fatal_signal_pending() and does not set JOBCTL_STOP_PENDING. The test-case above needs root and (correctly) crashes the kernel, but we can trigger the same warning inside the container or using another test-case: static int init(void *arg) { for (;;) pause(); } int main(void) { char stack[16 * 1024]; for (;;) { int pid = clone(init, stack + sizeof(stack)/2, CLONE_NEWPID | SIGCHLD, NULL); assert(pid > 0); assert(ptrace(PTRACE_ATTACH, pid, 0, 0) == 0); assert(waitpid(-1, NULL, WSTOPPED) == pid); assert(ptrace(PTRACE_DETACH, pid, 0, SIGSTOP) == 0); assert(syscall(__NR_tkill, pid, SIGKILL) == 0); assert(pid == wait(NULL)); } } Reported-by: Dmitry Vyukov Signed-off-by: Oleg Nesterov --- kernel/signal.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-) diff --git a/kernel/signal.c b/kernel/signal.c index d64efad..9b6f385 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -914,7 +914,7 @@ 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)) { /* -- 1.5.5.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/