linux-kernel - Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

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]

Date:   Thu, 7 Dec 2023 22:25:09 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Tycho Andersen <tycho@...ho.pizza>, Oleg Nesterov <oleg@...hat.com>
Cc:     "Eric W . Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Tycho Andersen <tandersen@...flix.com>,
        Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

> If these concerns are correct

So, ok. I misremebered this. The scenario I had been thinking of is
basically the following.

We have a thread-group with thread-group leader 1234 and a thread with
4567 in that thread-group. Assume current thread-group leader is tsk1
and the non-thread-group leader is tsk2. tsk1 uses struct pid *tg_pid
and tsk2 uses struct pid *t_pid. The struct pids look like this after
creation of both thread-group leader tsk1 and thread tsk2:

	TGID 1234				TID 4567 
	tg_pid[PIDTYPE_PID]  = tsk1		t_pid[PIDTYPE_PID]  = tsk2
	tg_pid[PIDTYPE_TGID] = tsk1		t_pid[PIDTYPE_TGID] = NULL

IOW, tsk2's struct pid has never been used as a thread-group leader and
thus PIDTYPE_TGID is NULL. Now assume someone does create pidfds for
tsk1 and for tsk2:

	tg_pidfd = pidfd_open(tsk1)		t_pidfd = pidfd_open(tsk2)
	-> tg_pidfd->private_data = tg_pid	-> t_pidfd->private_data = t_pid

So we stash away struct pid *tg_pid for a pidfd_open() on tsk1 and we
stash away struct pid *t_pid for a pidfd_open() on tsk2.

If we wait on that task via P_PIDFD we get:

				/* waiting through pidfd */
	waitid(P_PIDFD, tg_pidfd)		waitid(P_PIDFD, t_pidfd)
	tg_pid[PIDTYPE_TGID] == tsk1		t_pid[PIDTYPE_TGID] == NULL
	=> succeeds				=> fails

Because struct pid *tg_pid is used a thread-group leader struct pid we
can wait on that tsk1. But we can't via the non-thread-group leader
pidfd because the struct pid *t_pid has never been used as a
thread-group leader.

Now assume, t_pid exec's and the struct pids are transfered. IIRC, we
get:

	tg_pid[PIDTYPE_PID]   = tsk2		t_pid[PIDTYPE_PID]   = tsk1
	tg_pid[PIDTYPE_TGID]  = tsk2		t_pid[PIDTYPE_TGID]  = NULL

If we wait on that task via P_PIDFD we get:

				/* waiting through pidfd */
	waitid(P_PIDFD, tg_pidfd)		waitid(P_PIDFD, t_pid)
	tg_pid[PIDTYPE_TGID] == tsk2		t_pid[PIDTYPE_TGID] == NULL
	=> succeeds				=> fails

Which is what we want. So effectively this should all work and I
misremembered the struct pid linkage. So afaict we don't even have a
problem here which is great.