linux-kernel - Re: [PATCH] signal: Avoid corrupting si_pid and si_uid in do_notify

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Message-ID: <20200421102104.6pt34jknxmtu5ygm@wittgenstein>
Date:   Tue, 21 Apr 2020 12:21:04 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Christof Meerwald <cmeerw@...erw.org>
Subject: Re: [PATCH] signal: Avoid corrupting si_pid and si_uid in
 do_notify_parent

On Tue, Apr 21, 2020 at 11:28:47AM +0200, Oleg Nesterov wrote:
> On 04/21, Christian Brauner wrote:
> >
> > > -		__group_send_sig_info(sig, &info, tsk->parent);
> > > +		__send_signal(sig, &info, tsk->parent, PIDTYPE_TGID, false);
> >
> > So below you switch to __send_signal() but set the "force" argument to
> > to "false".
> 
> it must be false, the signal is generated from the parent's namespace or
> its descendant
> 
> > Before that, if the signal was generated from another pid
> > namespace and we fixed up si_pid and si_uid the "force" argument was set
> > to "true",
> 
> before that the "force" argument could be falsely true by the same reason,
> task_pid_nr_ns(tsk, tsk->parent) can return 0 because "tsk" no longer have
> pids after __unhash_process().

As I said in my mail, looking at the codepath it seems safe. My question
was whether it is _always_ incorrectly false which I do think it is
because child subreapers can't come from outside the pid namespace. If
they could you could create a scenario where the signal is generated
from a sibling pid namespace in which case it would be correctly set to
true.

Christian