linux-kernel - Re: SELinux lead to soft lockup when pid 1 proceess reap child

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Date:   Mon, 9 Jan 2017 19:12:25 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     yangshukui <yangshukui@...wei.com>
Cc:     selinux@...ho.nsa.gov, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        "Guohanjun (Hanjun Guo)" <guohanjun@...wei.com>,
        "'Qiang Huang'" <h.huangqiang@...wei.com>,
        Lizefan <lizefan@...wei.com>, "miaoxie (A)" <miaoxie@...wei.com>,
        Zhangdianfang <zhangdianfang@...wei.com>, paul@...l-moore.com,
        sds@...ho.nsa.gov, eparis@...isplace.org,
        james.l.morris@...cle.com, ebiederm@...ssion.com,
        serge.hallyn@...ntu.com
Subject: Re: SELinux lead to soft lockup when pid 1 proceess reap child

On 01/09, yangshukui wrote:
>
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3596,6 +3596,9 @@ static int selinux_task_kill(struct task_struct *p,
> struct siginfo *info,
>
>  static int selinux_task_wait(struct task_struct *p)
>  {
> +       if (pid_vnr(task_tgid(current)) == 1){
> +                return 0;

this check is not really correct, it can be a sub-thread... Doesn't matter,
please see below.

> +       }
>         return task_has_perm(p, current, PROCESS__SIGCHLD);
>  }
> It work but it permit pid 1 process to reap child without selinux check. Can
> we have a better way to handle this problem?

I never understood why security_task_wait() should deny to reap a child. But
since it can we probably want some explicit "the whole namespace goes away" check.
We could use, say, PIDNS_HASH_ADDING but I'd suggest something like a trivial change
below for now.

Eric, what do you think?

Oleg.

diff --git a/security/security.c b/security/security.c
index f825304..1330b4e 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1027,6 +1027,9 @@ int security_task_kill(struct task_struct *p, struct siginfo *info,
 
 int security_task_wait(struct task_struct *p)
 {
+	/* must be the exiting child reaper */
+	if (unlikely(current->flags & PF_EXITING))
+		return 0;
 	return call_int_hook(task_wait, 0, p);
 }