[PATCH 5.10 0/1] sched/fair: fix fault in reweight_entity()

[PATCH 5.10 0/1] sched/fair: fix fault in reweight_entity()

[Fedor Pchelkin]

Syzkaller reports general protection fault at reweight_entity() in 5.10
stable releases. The problem has been fixed by the following patch which
can be applied with minor changes to the 5.10 branch.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

[PATCH 5.10 1/1] sched/fair: Fix fault in reweight_entity

[Fedor Pchelkin]

From: Tadeusz Struk <tadeusz.struk@linaro.org>

commit 13765de8148f71fa795e0a6607de37c49ea5915a upstream.

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.

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 remove the update_load param from the update_load param() function
and call reweight_task() only if the task flag doesn't have the
TASK_NEW flag set.

Fixes: 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
Reported-by: syzbot+af7a719bc92395ee41b3@syzkaller.appspotmail.com
Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20220203161846.1160750-1-tadeusz.struk@linaro.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
---
5.10 adaptation: Replaced a task_struct field '__state' with 'state' 

 kernel/sched/core.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e437d946b27b..86c5e08b393b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -844,8 +844,9 @@ int tg_nop(struct task_group *tg, void *data)
 }
 #endif
 
-static void set_load_weight(struct task_struct *p, bool update_load)
+static void set_load_weight(struct task_struct *p)
 {
+	bool update_load = !(READ_ONCE(p->state) & TASK_NEW);
 	int prio = p->static_prio - MAX_RT_PRIO;
 	struct load_weight *load = &p->se.load;
 
@@ -3262,7 +3263,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 			p->static_prio = NICE_TO_PRIO(0);
 
 		p->prio = p->normal_prio = p->static_prio;
-		set_load_weight(p, false);
+		set_load_weight(p);
 
 		/*
 		 * We don't need the reset flag anymore after the fork. It has
@@ -5011,7 +5012,7 @@ 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);
 	old_prio = p->prio;
 	p->prio = effective_prio(p);
 
@@ -5184,7 +5185,7 @@ 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);
 }
 
 /*
@@ -7189,7 +7190,7 @@ void __init sched_init(void)
 		atomic_set(&rq->nr_iowait, 0);
 	}
 
-	set_load_weight(&init_task, false);
+	set_load_weight(&init_task);
 
 	/*
 	 * The boot idle thread does lazy MMU switching as well:
-- 
2.25.1

Re: [PATCH 5.10 1/1] sched/fair: Fix fault in reweight_entity

[Greg Kroah-Hartman]

On Fri, Aug 19, 2022 at 10:11:40AM +0300, Fedor Pchelkin wrote:
> From: Tadeusz Struk <tadeusz.struk@linaro.org>
> 
> commit 13765de8148f71fa795e0a6607de37c49ea5915a upstream.
> 
> 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.
> 
> 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 remove the update_load param from the update_load param() function
> and call reweight_task() only if the task flag doesn't have the
> TASK_NEW flag set.
> 
> Fixes: 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
> Reported-by: syzbot+af7a719bc92395ee41b3@syzkaller.appspotmail.com
> Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: stable@vger.kernel.org
> Link: https://lkml.kernel.org/r/20220203161846.1160750-1-tadeusz.struk@linaro.org
> Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
> ---
> 5.10 adaptation: Replaced a task_struct field '__state' with 'state' 

Now queued up, thanks.

greg k-h