linux-kernel - Re: [PATCH 1/1] signal: kill the obsolete SIGNAL_UNKILLABLE check in complete

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20151104172123.590fcbf0e8904911eeab3e3d@linux-foundation.org>
Date:	Wed, 4 Nov 2015 17:21:23 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	Roland McGrath <roland@...k.frob.com>, amanieu@...il.com,
	pmoore@...hat.com, Ingo Molnar <mingo@...nel.org>,
	vdavydov@...allels.com, qiaowei.ren@...el.com, dave@...olabs.net,
	palmer@...belt.com, LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [PATCH 1/1] signal: kill the obsolete SIGNAL_UNKILLABLE check
 in complete_signal()

On Wed, 4 Nov 2015 20:19:12 +0100 Oleg Nesterov <oleg@...hat.com> wrote:

> 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));
> 		}
> 	}

I'm thinking this should be backported into -stable due to WARN_ONs and
kernel crashes.  And as f008faff0e27 is from 2009, that means all
kernels.

Your thoughts on this?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/