linux-kernel - Maybe a race window in cgroup.kill?

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-next>] [day] [month] [year] [list]

Message-ID: <Z5QHE2Qn-QZ6M-KW@slm.duckdns.org>
Date: Fri, 24 Jan 2025 11:33:07 -1000
From: Tejun Heo <tj@...nel.org>
To: Christian Brauner <brauner@...nel.org>
Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	Michal Koutný <mkoutny@...e.com>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Joshua Hahn <joshua.hahnjy@...il.com>
Subject: Maybe a race window in cgroup.kill?

Hello, Christian.

I was looking at cgroup.kill implementation and wondering whether there
could be a race window. So, __cgroup_kill() does the following:

 k1. Set CGRP_KILL.
 k2. Iterate tasks and deliver SIGKILL.
 k3. Clear CGRP_KILL.

The copy_process() does the following:

 c1. Copy a bunch of stuff.
 c2. Grab siglock.
 c3. Check fatal_signal_pending().
 c4. Commit to forking.
 c5. Release siglock.
 c6. Call cgroup_post_fork() which puts the task on the css_set and tests
     CGRP_KILL.

The intention seems to be that either a forking task gets SIGKILL and
terminates on c3 or it sees CGRP_KILL on c6 and kills the child. However, I
don't see what guarantees that k3 can't happen before c6. ie. After a
forking task passes c5, k2 can take place and then before the forking task
reaches c6, k3 can happen. Then, nobody would send SIGKILL to the child.
What am I missing?

Thanks.

-- 
tejun