linux-kernel - Re: [PATCH v2 00/15] Make the user mode driver code a better citizen

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Date:   Tue, 30 Jun 2020 07:29:34 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <greg@...ah.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>, bpf <bpf@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Gary Lin <GLin@...e.com>, Bruno Meneguele <bmeneg@...hat.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 00/15] Make the user mode driver code a better citizen

Alexei Starovoitov <alexei.starovoitov@...il.com> writes:

2> On Mon, Jun 29, 2020 at 02:55:05PM -0500, Eric W. Biederman wrote:
>> 
>> I have tested thes changes by booting with the code compiled in and
>> by killing "bpfilter_umh" and running iptables -vnL to restart
>> the userspace driver.
>> 
>> I have compiled tested each change with and without CONFIG_BPFILTER
>> enabled.
>
> With
> CONFIG_BPFILTER=y
> CONFIG_BPFILTER_UMH=m
> it doesn't build:
>
> ERROR: modpost: "kill_pid_info" [net/bpfilter/bpfilter.ko] undefined!
>
> I've added:
> +EXPORT_SYMBOL(kill_pid_info);
> to continue testing...

I am rather surprised I thought Tetsuo had already compile tested
modules.


> I suspect patch 13 is somehow responsible:
> +	if (tgid) {
> +		kill_pid_info(SIGKILL, SEND_SIG_PRIV, tgid);
> +		wait_event(tgid->wait_pidfd, !pid_task(tgid, PIDTYPE_TGID));
> +		bpfilter_umh_cleanup(info);
> +	}
>
> I cannot figure out why it hangs. Some sort of race ?
> Since adding short delay between kill and wait makes it work.

Having had a chance to sleep kill_pid_info was a thinko, as was
!pid_task.  It should have been !pid_has_task as that takes the proper
rcu locking.

I don't know if that is going to be enough to fix the wait_event
but those are obvious bugs that need to be fixed.

diff --git a/net/bpfilter/bpfilter_kern.c b/net/bpfilter/bpfilter_kern.c
index 91474884ddb7..3e1874030daa 100644
--- a/net/bpfilter/bpfilter_kern.c
+++ b/net/bpfilter/bpfilter_kern.c
@@ -19,8 +19,8 @@ static void shutdown_umh(void)
        struct pid *tgid = info->tgid;
 
        if (tgid) {
-               kill_pid_info(SIGKILL, SEND_SIG_PRIV, tgid);
-               wait_event(tgid->wait_pidfd, !pid_task(tgid, PIDTYPE_TGID));
+               kill_pid(tgid, SIGKILL, 1);
+               wait_event(tgid->wait_pidfd, !pid_has_task(tgid, PIDTYPE_TGID));
                bpfilter_umh_cleanup(info);
        }
 }

> And then did:
> while true; do iptables -L;rmmod bpfilter; done
>  
> Unfortunately sometimes 'rmmod bpfilter' hangs in wait_event().

Hmm.  The wake up happens just of tgid->wait_pidfd happens just before
release_task is called so there is a race.  As it is possible to wake
up and then go back to sleep before pid_has_task becomes false.

So I think I need a friendly helper that does:

bool task_has_exited(struct pid *tgid)
{
	bool exited = false;

	rcu_read_lock();
        tsk = pid_task(tgid, PIDTYPE_TGID);
        exited = !!tsk;
        if (tsk) {
        	exited = !!tsk->exit_state;
out:
	rcu_unlock();
	return exited;
}

There should be a sensible way to do that.

Eric