Date:   Tue, 25 Jan 2022 11:34:03 -0800
From:   Tadeusz Struk <tadeusz.struk@...aro.org>
To:     peterz@...radead.org
Cc:     Tadeusz Struk <tadeusz.struk@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Zhang Qiao <zhangqiao22@...wei.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+af7a719bc92395ee41b3@...kaller.appspotmail.com
Subject: [PATCH v3] sched/fair: Fix fault in reweight_entity

Syzbot found a GPF in reweight_entity. This has been bisected to commit
4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")

There is a race between sched_post_fork() and setpriority(PRIO_PGRP)
within a thread group that causes a null-ptr-deref in reweight_entity()
in CFS. The scenario is that the main process spawns number of new
threads, which then call setpriority(PRIO_PGRP, 0, -20), wait, and exit.
For each of the new threads the copy_process() gets invoked, which adds
the new task_struct and calls sched_post_fork() for it.

In the above scenario there is a possibility that setpriority(PRIO_PGRP)
and set_one_prio() will be called for a thread in the group that is just
being created by copy_process(), and for which the sched_post_fork() has
not been executed yet. This will trigger a null pointer dereference in
reweight_entity(), as it will try to access the run queue pointer, which
hasn't been set. This results it a crash as shown below:

KASAN: null-ptr-deref in range [0x00000000000000a0-0x00000000000000a7]
CPU: 0 PID: 2392 Comm: reduced_repro Not tainted 5.16.0-11201-gb42c5a161ea3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1.fc35 04/01/2014
RIP: 0010:reweight_entity+0x15d/0x440
RSP: 0018:ffffc900035dfcf8 EFLAGS: 00010006
Call Trace:
<TASK>
reweight_task+0xde/0x1c0
set_load_weight+0x21c/0x2b0
set_user_nice.part.0+0x2d1/0x519
set_user_nice.cold+0x8/0xd
set_one_prio+0x24f/0x263
__do_sys_setpriority+0x2d3/0x640
__x64_sys_setpriority+0x84/0x8b
do_syscall_64+0x35/0xb0
entry_SYSCALL_64_after_hwframe+0x44/0xae
</TASK>
---[ end trace 9dc80a9d378ed00a ]---

Before the mentioned change the cfs_rq pointer for the task  has been
set in sched_fork(), which is called much earlier in copy_process(),
before the new task is added to the thread_group.
Now it is done in the sched_post_fork(), which is called after that.
To fix the issue the update_load condition passed to set_load_weight()
in set_user_nice() and __sched_setscheduler() has been changed from
always true to true if the task->state != TASK_NEW.

Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Zhang Qiao <zhangqiao22@...wei.com>
Cc: Daniel Jordan <daniel.m.jordan@...cle.com>
Cc: stable@...r.kernel.org
Cc: linux-kernel@...r.kernel.org

Link: https://syzkaller.appspot.com/bug?id=9d9c27adc674e3a7932b22b61c79a02da82cbdc1
Fixes: 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
Reported-by: syzbot+af7a719bc92395ee41b3@...kaller.appspotmail.com
Signed-off-by: Tadeusz Struk <tadeusz.struk@...aro.org>
---
Changes in v3:
- Removed the new check and changed the update_load condition from
  always true to true if p->state != TASK_NEW

Changes in v2:
- Added a check in set_user_nice(), and return from there if the task
  is not fully setup instead of returning from reweight_entity()
---
---
 kernel/sched/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 848eaa0efe0e..3d7ede06b971 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6921,7 +6921,7 @@ void set_user_nice(struct task_struct *p, long nice)
 		put_prev_task(rq, p);
 
 	p->static_prio = NICE_TO_PRIO(nice);
-	set_load_weight(p, true);
+	set_load_weight(p, !(READ_ONCE(p->__state) & TASK_NEW));
 	old_prio = p->prio;
 	p->prio = effective_prio(p);
 
@@ -7212,7 +7212,7 @@ static void __setscheduler_params(struct task_struct *p,
 	 */
 	p->rt_priority = attr->sched_priority;
 	p->normal_prio = normal_prio(p);
-	set_load_weight(p, true);
+	set_load_weight(p, !(READ_ONCE(p->__state) & TASK_NEW));
 }
 
 /*
-- 
2.34.1

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