Re: [syzbot] [cgroups?] WARNING in css_rstat_exit

[JP Kobryn <inwardvessel@gmail.com>]

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#syz test

On 7/14/25 12:29 AM, syzbot wrote:
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    5d5d62298b8b Merge tag 'x86_urgent_for_v6.16_rc6' of git:/..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11dabd82580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=84eae426cbd8669c
> dashboard link: https://syzkaller.appspot.com/bug?extid=8d052e8b99e40bc625ed
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=162c47d4580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/d2d0d46a0e87/disk-5d5d6229.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/0bf6381177a8/vmlinux-5d5d6229.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/2f3ae8f165f2/bzImage-5d5d6229.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+8d052e8b99e40bc625ed@syzkaller.appspotmail.com
> 
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 9 at kernel/cgroup/rstat.c:497 css_rstat_exit+0x368/0x470 kernel/cgroup/rstat.c:497
> Modules linked in:
> CPU: 0 UID: 0 PID: 9 Comm: kworker/0:0 Not tainted 6.16.0-rc5-syzkaller-00276-g5d5d62298b8b #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
> Workqueue: cgroup_destroy css_free_rwork_fn
> RIP: 0010:css_rstat_exit+0x368/0x470 kernel/cgroup/rstat.c:497
> Code: 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 0f 85 0e 01 00 00 49 c7 85 70 05 00 00 00 00 00 00 e9 00 ff ff ff e8 d9 09 07 00 90 <0f> 0b 90 e9 3e ff ff ff e8 cb 09 07 00 90 0f 0b 90 e9 30 ff ff ff
> RSP: 0018:ffffc900000e7bc0 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: ffff8881404a4e00 RCX: ffff888124720000
> RDX: ffff88801e298000 RSI: ffffffff81b45507 RDI: ffffffff8df37da0
> RBP: ffff8881404a4e08 R08: 0000000000000005 R09: 0000000000000007
> R10: 0000000000000000 R11: 0000000000000001 R12: ffff8881404a4e20
> R13: 0000000000000000 R14: 0000000000000003 R15: dffffc0000000000
> FS:  0000000000000000(0000) GS:ffff888124720000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000000 CR3: 0000000034892000 CR4: 00000000003526f0
> Call Trace:
>   <TASK>
>   css_free_rwork_fn+0x80/0x12e0 kernel/cgroup/cgroup.c:5449
>   process_one_work+0x9cf/0x1b70 kernel/workqueue.c:3238
>   process_scheduled_works kernel/workqueue.c:3321 [inline]
>   worker_thread+0x6c8/0xf10 kernel/workqueue.c:3402
>   kthread+0x3c2/0x780 kernel/kthread.c:464
>   ret_from_fork+0x5d7/0x6f0 arch/x86/kernel/process.c:148
>   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>   </TASK>
> 
> 
> ---
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.

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From 381235b9f2aa500b6e2971e98ea84edc107cd1d8 Mon Sep 17 00:00:00 2001
From: JP Kobryn <inwardvessel@gmail.com>
Date: Mon, 14 Jul 2025 16:45:55 -0700
Subject: [PATCH] cgroup: make sure css_rstat_init() is called before css_rstat_exit()

Test against syzbot repro.

Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
---
 kernel/cgroup/cgroup.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index a723b7dc6e4e..e6c5c998ead6 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5669,6 +5669,12 @@ static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
 
 	init_and_link_css(css, ss, cgrp);
 
+	err = css_rstat_init(css);
+	if (err) {
+		ss->css_free(css);
+		goto err_out;
+	}
+
 	err = percpu_ref_init(&css->refcnt, css_release, 0, GFP_KERNEL);
 	if (err)
 		goto err_free_css;
@@ -5678,10 +5684,6 @@ static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
 		goto err_free_css;
 	css->id = err;
 
-	err = css_rstat_init(css);
-	if (err)
-		goto err_free_css;
-
 	/* @css is ready to be brought online now, make it visible */
 	list_add_tail_rcu(&css->sibling, &parent_css->children);
 	cgroup_idr_replace(&ss->css_idr, css, css->id);
@@ -5697,6 +5699,7 @@ static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
 err_free_css:
 	INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
 	queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
+err_out:
 	return ERR_PTR(err);
 }
 
-- 
2.47.1