[PATCH v2] sched/fair: Fix fault in reweight_entity

[Tadeusz Struk]

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, prio), wait, and exit.
For each of the new threads the copy_process() gets invoked, which adds
the new task_struct to the group, and eventually 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.

Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx

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@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Tadeusz Struk <tadeusz.struk@xxxxxxxxxx>
---
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 ++++
 kernel/sched/sched.h | 11 +++++++++++
 2 files changed, 15 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2e4ae00e52d1..c3e74b6d595b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6897,6 +6897,10 @@ void set_user_nice(struct task_struct *p, long nice)
 
 	if (task_nice(p) == nice || nice < MIN_NICE || nice > MAX_NICE)
 		return;
+
+	/* Check if the task's schedule run queue is setup correctly */
+	if (!task_rq_ready(p))
+		return;
 	/*
 	 * We have to be careful, if called from sys_setpriority(),
 	 * the task might be in the middle of scheduling on another CPU.
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index de53be905739..464f629bff5a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1394,6 +1394,12 @@ static inline struct cfs_rq *group_cfs_rq(struct sched_entity *grp)
 	return grp->my_q;
 }
 
+/* returns true if cfs run queue is set for the task */
+static inline bool task_rq_ready(struct task_struct *p)
+{
+	return !!task_cfs_rq(p);
+}
+
 #else
 
 static inline struct task_struct *task_of(struct sched_entity *se)
@@ -1419,6 +1425,11 @@ static inline struct cfs_rq *group_cfs_rq(struct sched_entity *grp)
 {
 	return NULL;
 }
+
+static inline bool task_rq_ready(struct task_struct *p)
+{
+	return true;
+}
 #endif
 
 extern void update_rq_clock(struct rq *rq);
-- 
2.34.1