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Subject[PATCH cgroup/for-4.15] cgroup: statically initialize init_css_set->dfl_cgrp
Hello,

A follow-up patch to fix sporadic oops during boot which is caused by
trying dereference the default cgroup from accounting code before
cgroup is initialized.

Applied to cgroup/for-4.15.

Thanks.
------ 8< ------
From 38683148828165ea0b66ace93a9fedc2d3281e27 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Mon, 25 Sep 2017 13:50:20 -0700
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Like other csets, init_css_set's dfl_cgrp is initialized when the cset
gets linked. init_css_set gets linked in cgroup_init(). This has
been fine till now but the recently added basic CPU usage accounting
may end up accessing dfl_cgrp of init before cgroup_init() leading to
the following oops.

SELinux: Initializing.
BUG: unable to handle kernel NULL pointer dereference at 00000000000000b0
IP: account_system_index_time+0x60/0x90
PGD 0 P4D 0
Oops: 0000 [#1] SMP
Modules linked in:
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.14.0-rc2-00003-g041cd64 #10
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
+1.9.3-20161025_171302-gandalf 04/01/2014
task: ffffffff81e10480 task.stack: ffffffff81e00000
RIP: 0010:account_system_index_time+0x60/0x90
RSP: 0000:ffff880011e03cb8 EFLAGS: 00010002
RAX: ffffffff81ef8800 RBX: ffffffff81e10480 RCX: 0000000000000003
RDX: 0000000000000000 RSI: 00000000000f4240 RDI: 0000000000000000
RBP: ffff880011e03cc0 R08: 0000000000010000 R09: 0000000000000000
R10: 0000000000000020 R11: 0000003b9aca0000 R12: 000000000001c100
R13: 0000000000000000 R14: ffffffff81e10480 R15: ffffffff81e03cd8
FS: 0000000000000000(0000) GS:ffff880011e00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000000000b0 CR3: 0000000001e09000 CR4: 00000000000006b0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<IRQ>
account_system_time+0x45/0x60
account_process_tick+0x5a/0x140
update_process_times+0x22/0x60
tick_periodic+0x2b/0x90
tick_handle_periodic+0x25/0x70
timer_interrupt+0x15/0x20
__handle_irq_event_percpu+0x7e/0x1b0
handle_irq_event_percpu+0x23/0x60
handle_irq_event+0x42/0x70
handle_level_irq+0x83/0x100
handle_irq+0x6f/0x110
do_IRQ+0x46/0xd0
common_interrupt+0x9d/0x9d

Fix it by statically initializing init_css_set.dfl_cgrp so that init's
default cgroup is accessible from the get-go.

Fixes: 041cd640b2f3 ("cgroup: Implement cgroup2 basic CPU usage accounting")
Reported-by: “kbuild-all@01.org” <kbuild-all@01.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/cgroup/cgroup.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index d036625..7975b20 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -649,6 +649,14 @@ struct css_set init_css_set = {
.cgrp_links = LIST_HEAD_INIT(init_css_set.cgrp_links),
.mg_preload_node = LIST_HEAD_INIT(init_css_set.mg_preload_node),
.mg_node = LIST_HEAD_INIT(init_css_set.mg_node),
+
+ /*
+ * The following field is re-initialized when this cset gets linked
+ * in cgroup_init(). However, let's initialize the field
+ * statically too so that the default cgroup can be accessed safely
+ * early during boot.
+ */
+ .dfl_cgrp = &cgrp_dfl_root.cgrp,
};

static int css_set_count = 1; /* 1 for init_css_set */
--
2.9.5
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