[PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Zhang Qiao]

There is a small race between copy_process() and sched_fork()
where child->sched_task_group point to an already freed pointer.

parent doing fork()      | someone moving the parent
				to another cgroup
-------------------------------+-------------------------------
copy_process()
    + dup_task_struct()<1>
				parent move to another cgroup,
				and free the old cgroup. <2>
    + sched_fork()
      + __set_task_cpu()<3>
      + task_fork_fair()
        + sched_slice()<4>

In the worst case, this bug can lead to "use-after-free" and
cause panic as shown above,
(1)parent copy its sched_task_group to child at <1>;
(2)someone move the parent to another cgroup and free the old
   cgroup at <2>;
(3)the sched_task_group and cfs_rq that belong to the old cgroup
   will be accessed at <3> and <4>, which cause a panic:

[89249.732198] BUG: unable to handle kernel NULL pointer
dereference at 0000000000000000
[89249.732701] PGD 8000001fa0a86067 P4D 8000001fa0a86067 PUD
2029955067 PMD 0
[89249.733005] Oops: 0000 [#1] SMP PTI
[89249.733288] CPU: 7 PID: 648398 Comm: ebizzy Kdump: loaded
Tainted: G           OE    --------- -  - 4.18.0.x86_64+ #1
[89249.734318] RIP: 0010:sched_slice+0x84/0xc0
 ....
[89249.737910] Call Trace:
[89249.738181]  task_fork_fair+0x81/0x120
[89249.738457]  sched_fork+0x132/0x240
[89249.738732]  copy_process.part.5+0x675/0x20e0
[89249.739010]  ? __handle_mm_fault+0x63f/0x690
[89249.739286]  _do_fork+0xcd/0x3b0
[89249.739558]  do_syscall_64+0x5d/0x1d0
[89249.739830]  entry_SYSCALL_64_after_hwframe+0x65/0xca
[89249.740107] RIP: 0033:0x7f04418cd7e1

When a new process is forked, cgroup_post_fork() associates it
with the cgroup of its parent. Therefore this commit move the
__set_task_cpu() and task_fork() that access some cgroup-related
fields(sched_task_group and cfs_rq) to sched_post_fork() and
call sched_post_fork() after cgroup_post_fork().

Fixes: 8323f26ce342 ("sched: Fix race in task_group")
Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
---
 kernel/fork.c       |  2 +-
 kernel/sched/core.c | 33 +++++++++++++--------------------
 2 files changed, 14 insertions(+), 21 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index bc94b2cc5995..a4acd87f00f9 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2330,8 +2330,8 @@ static __latent_entropy struct task_struct *copy_process(
 	write_unlock_irq(&tasklist_lock);
 
 	proc_fork_connector(p);
-	sched_post_fork(p);
 	cgroup_post_fork(p, args);
+	sched_post_fork(p);
 	perf_event_fork(p);
 
 	trace_task_newtask(p, clone_flags);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 20ffcc044134..041f21e8b266 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4069,8 +4069,6 @@ int sysctl_schedstats(struct ctl_table *table, int write, void *buffer,
  */
 int sched_fork(unsigned long clone_flags, struct task_struct *p)
 {
-	unsigned long flags;
-
 	__sched_fork(clone_flags, p);
 	/*
 	 * We mark the process as NEW here. This guarantees that
@@ -4116,24 +4114,6 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 
 	init_entity_runnable_average(&p->se);
 
-	/*
-	 * The child is not yet in the pid-hash so no cgroup attach races,
-	 * and the cgroup is pinned to this child due to cgroup_fork()
-	 * is ran before sched_fork().
-	 *
-	 * Silence PROVE_RCU.
-	 */
-	raw_spin_lock_irqsave(&p->pi_lock, flags);
-	rseq_migrate(p);
-	/*
-	 * We're setting the CPU for the first time, we don't migrate,
-	 * so use __set_task_cpu().
-	 */
-	__set_task_cpu(p, smp_processor_id());
-	if (p->sched_class->task_fork)
-		p->sched_class->task_fork(p);
-	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
-
 #ifdef CONFIG_SCHED_INFO
 	if (likely(sched_info_on()))
 		memset(&p->sched_info, 0, sizeof(p->sched_info));
@@ -4151,6 +4131,19 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 
 void sched_post_fork(struct task_struct *p)
 {
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&p->pi_lock, flags);
+	rseq_migrate(p);
+	/*
+	 * We're setting the CPU for the first time, we don't migrate,
+	 * so use __set_task_cpu().
+	 */
+	__set_task_cpu(p, smp_processor_id());
+	if (p->sched_class->task_fork)
+		p->sched_class->task_fork(p);
+	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+
 	uclamp_post_fork(p);
 }
 
-- 
2.17.1

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Zhang Qiao]

On 2021/8/26 19:26, Zhang Qiao wrote:
> There is a small race between copy_process() and sched_fork()
> where child->sched_task_group point to an already freed pointer.
> 
> parent doing fork()      | someone moving the parent
> 				to another cgroup
> -------------------------------+-------------------------------
> copy_process()
>     + dup_task_struct()<1>
> 				parent move to another cgroup,
> 				and free the old cgroup. <2>
>     + sched_fork()
>       + __set_task_cpu()<3>
>       + task_fork_fair()
>         + sched_slice()<4>
> 
> In the worst case, this bug can lead to "use-after-free" and
> cause panic as shown above,
> (1)parent copy its sched_task_group to child at <1>;
> (2)someone move the parent to another cgroup and free the old
>    cgroup at <2>;
> (3)the sched_task_group and cfs_rq that belong to the old cgroup
>    will be accessed at <3> and <4>, which cause a panic:
> 
> [89249.732198] BUG: unable to handle kernel NULL pointer
> dereference at 0000000000000000
> [89249.732701] PGD 8000001fa0a86067 P4D 8000001fa0a86067 PUD
> 2029955067 PMD 0
> [89249.733005] Oops: 0000 [#1] SMP PTI
> [89249.733288] CPU: 7 PID: 648398 Comm: ebizzy Kdump: loaded
> Tainted: G           OE    --------- -  - 4.18.0.x86_64+ #1
> [89249.734318] RIP: 0010:sched_slice+0x84/0xc0
>  ....
> [89249.737910] Call Trace:
> [89249.738181]  task_fork_fair+0x81/0x120
> [89249.738457]  sched_fork+0x132/0x240
> [89249.738732]  copy_process.part.5+0x675/0x20e0
> [89249.739010]  ? __handle_mm_fault+0x63f/0x690
> [89249.739286]  _do_fork+0xcd/0x3b0
> [89249.739558]  do_syscall_64+0x5d/0x1d0
> [89249.739830]  entry_SYSCALL_64_after_hwframe+0x65/0xca
> [89249.740107] RIP: 0033:0x7f04418cd7e1
> 
> When a new process is forked, cgroup_post_fork() associates it
> with the cgroup of its parent. Therefore this commit move the
> __set_task_cpu() and task_fork() that access some cgroup-related
> fields(sched_task_group and cfs_rq) to sched_post_fork() and
> call sched_post_fork() after cgroup_post_fork().
> 
> Fixes: 8323f26ce342 ("sched: Fix race in task_group")
> Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
hello, a gentle ping to confirm whether this gets a review or not.

thanks,

Qiao.

> ---
>  kernel/fork.c       |  2 +-
>  kernel/sched/core.c | 33 +++++++++++++--------------------
>  2 files changed, 14 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index bc94b2cc5995..a4acd87f00f9 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2330,8 +2330,8 @@ static __latent_entropy struct task_struct *copy_process(
>  	write_unlock_irq(&tasklist_lock);
>  
>  	proc_fork_connector(p);
> -	sched_post_fork(p);
>  	cgroup_post_fork(p, args);
> +	sched_post_fork(p);
>  	perf_event_fork(p);
>  
>  	trace_task_newtask(p, clone_flags);
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 20ffcc044134..041f21e8b266 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4069,8 +4069,6 @@ int sysctl_schedstats(struct ctl_table *table, int write, void *buffer,
>   */
>  int sched_fork(unsigned long clone_flags, struct task_struct *p)
>  {
> -	unsigned long flags;
> -
>  	__sched_fork(clone_flags, p);
>  	/*
>  	 * We mark the process as NEW here. This guarantees that
> @@ -4116,24 +4114,6 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
>  
>  	init_entity_runnable_average(&p->se);
>  
> -	/*
> -	 * The child is not yet in the pid-hash so no cgroup attach races,
> -	 * and the cgroup is pinned to this child due to cgroup_fork()
> -	 * is ran before sched_fork().
> -	 *
> -	 * Silence PROVE_RCU.
> -	 */
> -	raw_spin_lock_irqsave(&p->pi_lock, flags);
> -	rseq_migrate(p);
> -	/*
> -	 * We're setting the CPU for the first time, we don't migrate,
> -	 * so use __set_task_cpu().
> -	 */
> -	__set_task_cpu(p, smp_processor_id());
> -	if (p->sched_class->task_fork)
> -		p->sched_class->task_fork(p);
> -	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> -
>  #ifdef CONFIG_SCHED_INFO
>  	if (likely(sched_info_on()))
>  		memset(&p->sched_info, 0, sizeof(p->sched_info));
> @@ -4151,6 +4131,19 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
>  
>  void sched_post_fork(struct task_struct *p)
>  {
> +	unsigned long flags;
> +
> +	raw_spin_lock_irqsave(&p->pi_lock, flags);
> +	rseq_migrate(p);
> +	/*
> +	 * We're setting the CPU for the first time, we don't migrate,
> +	 * so use __set_task_cpu().
> +	 */
> +	__set_task_cpu(p, smp_processor_id());
> +	if (p->sched_class->task_fork)
> +		p->sched_class->task_fork(p);
> +	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> +
>  	uclamp_post_fork(p);
>  }
>  
> 

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Peter Zijlstra]

On Thu, Aug 26, 2021 at 07:26:35PM +0800, Zhang Qiao wrote:
> There is a small race between copy_process() and sched_fork()
> where child->sched_task_group point to an already freed pointer.
> 
> parent doing fork()      | someone moving the parent
> 				to another cgroup
> -------------------------------+-------------------------------
> copy_process()
>     + dup_task_struct()<1>
> 				parent move to another cgroup,
> 				and free the old cgroup. <2>
>     + sched_fork()
>       + __set_task_cpu()<3>
>       + task_fork_fair()
>         + sched_slice()<4>
> 
> In the worst case, this bug can lead to "use-after-free" and
> cause panic as shown above,
> (1)parent copy its sched_task_group to child at <1>;
> (2)someone move the parent to another cgroup and free the old
>    cgroup at <2>;
> (3)the sched_task_group and cfs_rq that belong to the old cgroup
>    will be accessed at <3> and <4>, which cause a panic:
> 
> [89249.732198] BUG: unable to handle kernel NULL pointer
> dereference at 0000000000000000
> [89249.732701] PGD 8000001fa0a86067 P4D 8000001fa0a86067 PUD
> 2029955067 PMD 0
> [89249.733005] Oops: 0000 [#1] SMP PTI
> [89249.733288] CPU: 7 PID: 648398 Comm: ebizzy Kdump: loaded
> Tainted: G           OE    --------- -  - 4.18.0.x86_64+ #1
> [89249.734318] RIP: 0010:sched_slice+0x84/0xc0
>  ....
> [89249.737910] Call Trace:
> [89249.738181]  task_fork_fair+0x81/0x120
> [89249.738457]  sched_fork+0x132/0x240
> [89249.738732]  copy_process.part.5+0x675/0x20e0
> [89249.739010]  ? __handle_mm_fault+0x63f/0x690
> [89249.739286]  _do_fork+0xcd/0x3b0
> [89249.739558]  do_syscall_64+0x5d/0x1d0
> [89249.739830]  entry_SYSCALL_64_after_hwframe+0x65/0xca
> [89249.740107] RIP: 0033:0x7f04418cd7e1
> 
> When a new process is forked, cgroup_post_fork() associates it
> with the cgroup of its parent. Therefore this commit move the
> __set_task_cpu() and task_fork() that access some cgroup-related
> fields(sched_task_group and cfs_rq) to sched_post_fork() and
> call sched_post_fork() after cgroup_post_fork().
> 
> Fixes: 8323f26ce342 ("sched: Fix race in task_group")
> Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>

Hmm, I think you're right. Did something recently chagne in cgroup land
to make this more visible? This code hasn't changed in like 9 years.

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Tejun Heo]

Hello,

On Mon, Aug 30, 2021 at 04:39:54PM +0200, Peter Zijlstra wrote:
> > When a new process is forked, cgroup_post_fork() associates it
> > with the cgroup of its parent. Therefore this commit move the
> > __set_task_cpu() and task_fork() that access some cgroup-related
> > fields(sched_task_group and cfs_rq) to sched_post_fork() and
> > call sched_post_fork() after cgroup_post_fork().

I think this would allow cgroup migrations to take place before
sched_post_fork() is run, which likely will break stuff. The right
thing to do likely is taking sched_task_group (and whatever other
fields) after cgroup_can_fork(), which fixates the cgroup memberships,
is run. For other controllers, operations like this would be performed
from cgroup_subsys->fork() callback but it's tricky for sched due to
autogroup.

> > Fixes: 8323f26ce342 ("sched: Fix race in task_group")
> > Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
> 
> Hmm, I think you're right. Did something recently chagne in cgroup land
> to make this more visible? This code hasn't changed in like 9 years.

I can't think of any remotely recent change either. I guess ppl just
don't try to migrate the parent while fork is in progress.

Thanks.

-- 
tejun

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Zhang Qiao]

hi, thanks for your reviews.

On 2021/8/31 1:32, Tejun Heo wrote:
> Hello,
> 
> On Mon, Aug 30, 2021 at 04:39:54PM +0200, Peter Zijlstra wrote:
>>> When a new process is forked, cgroup_post_fork() associates it
>>> with the cgroup of its parent. Therefore this commit move the
>>> __set_task_cpu() and task_fork() that access some cgroup-related
>>> fields(sched_task_group and cfs_rq) to sched_post_fork() and
>>> call sched_post_fork() after cgroup_post_fork().
> 
> I think this would allow cgroup migrations to take place before
> sched_post_fork() is run, which likely will break stuff. The right
> thing to do likely is taking sched_task_group (and whatever other
> fields) after cgroup_can_fork(), which fixates the cgroup memberships,
But it still seems possible that it accessed an invalid sched_task_group?
because the child process does not update its sched_task_group util
cgroup_post_fork().
> is run. For other controllers, operations like this would be performed
> from cgroup_subsys->fork() callback but it's tricky for sched due to
> autogroup.
> 
>>> Fixes: 8323f26ce342 ("sched: Fix race in task_group")
>>> Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
>>
>> Hmm, I think you're right. Did something recently chagne in cgroup land
>> to make this more visible? This code hasn't changed in like 9 years.
I think this problem has always existed. I've reproduced it in multiple versions,
including 3.10 and 5.14-rc3.

> 
> I can't think of any remotely recent change either. I guess ppl just
> don't try to migrate the parent while fork is in progress.
> 
> Thanks.
> 

thandks.

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Tejun Heo]

Hello,

On Tue, Aug 31, 2021 at 03:58:42PM +0800, Zhang Qiao wrote:
> > I think this would allow cgroup migrations to take place before
> > sched_post_fork() is run, which likely will break stuff. The right
> > thing to do likely is taking sched_task_group (and whatever other
> > fields) after cgroup_can_fork(), which fixates the cgroup memberships,
>
> But it still seems possible that it accessed an invalid sched_task_group?
> because the child process does not update its sched_task_group util
> cgroup_post_fork().

Between cgroup_can_fork() and cgroup_post_fork(), the cgroup membership and
thus sched_task_group can't change, so if the child sets it to the parent's
inbetween, the sched_task_group can't go away.

Thanks.

-- 
tejun

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Zhang Qiao]

hello,tejun.

在 2021/9/1 6:59, Tejun Heo 写道:
> Hello,
> 
> On Tue, Aug 31, 2021 at 03:58:42PM +0800, Zhang Qiao wrote:
>>> I think this would allow cgroup migrations to take place before
>>> sched_post_fork() is run, which likely will break stuff. The right

cgroup migrations? Do you mean child process set its cgroups at
cgroup_subsys->fork()?

>>> thing to do likely is taking sched_task_group (and whatever other
>>> fields) after cgroup_can_fork(), which fixates the cgroup memberships,
>>
>> But it still seems possible that it accessed an invalid sched_task_group?
>> because the child process does not update its sched_task_group util
>> cgroup_post_fork().
> 
> Between cgroup_can_fork() and cgroup_post_fork(), the cgroup membership and
> thus sched_task_group can't change, so if the child sets it to the parent's
> inbetween, the sched_task_group can't go away.

so the child just need to update its sched_task_group after cgroup_can_fork(),
then call sched_fork(), is it right?

thanks.

--
Qiao Zhang.
> 
> Thanks.
> 

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Tejun Heo]

Hello,

On Wed, Sep 01, 2021 at 03:43:00PM +0800, Zhang Qiao wrote:
> 在 2021/9/1 6:59, Tejun Heo 写道:
> > On Tue, Aug 31, 2021 at 03:58:42PM +0800, Zhang Qiao wrote:
> >>> I think this would allow cgroup migrations to take place before
> >>> sched_post_fork() is run, which likely will break stuff. The right
> 
> cgroup migrations? Do you mean child process set its cgroups at
> cgroup_subsys->fork()?

As soon as cgroup_post_fork() is complete, userspace can try moving the
process to a different cgroup which can get confusing for sched_post_fork.

> > Between cgroup_can_fork() and cgroup_post_fork(), the cgroup membership and
> > thus sched_task_group can't change, so if the child sets it to the parent's
> > inbetween, the sched_task_group can't go away.
> 
> so the child just need to update its sched_task_group after cgroup_can_fork(),
> then call sched_fork(), is it right?

Yeah, the bug here is that a field which is dependent on cgroup membership
is being read before the cgroup membership is fixated.

Thanks.

-- 
tejun

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Zhang Qiao]

在 2021/9/2 0:45, Tejun Heo 写道:
> Hello,
> 
> On Wed, Sep 01, 2021 at 03:43:00PM +0800, Zhang Qiao wrote:
>> 在 2021/9/1 6:59, Tejun Heo 写道:
>>> On Tue, Aug 31, 2021 at 03:58:42PM +0800, Zhang Qiao wrote:
>>>>> I think this would allow cgroup migrations to take place before
>>>>> sched_post_fork() is run, which likely will break stuff. The right
>>
>> cgroup migrations? Do you mean child process set its cgroups at
>> cgroup_subsys->fork()?
> 
> As soon as cgroup_post_fork() is complete, userspace can try moving the
> process to a different cgroup which can get confusing for sched_post_fork.

hello,tejun

I checked the code again.
I don't quite understand what you said, if the child be moved between
cgroup_post_fork() and sched_post_sched(), what problems might it cause?
Does the child process will use the old sched_task_group at sched_post_fork()-->__set_task_cpu()?
Or there are other problems, Can you talk in more detail?

thanks!

---
Qiao Zhang
> 
>>> Between cgroup_can_fork() and cgroup_post_fork(), the cgroup membership and
>>> thus sched_task_group can't change, so if the child sets it to the parent's
>>> inbetween, the sched_task_group can't go away.
>>
>> so the child just need to update its sched_task_group after cgroup_can_fork(),
>> then call sched_fork(), is it right?
> 
> Yeah, the bug here is that a field which is dependent on cgroup membership
> is being read before the cgroup membership is fixated.
> 
> Thanks.
> 

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Tejun Heo]

Hello,

On Thu, Sep 02, 2021 at 03:42:15PM +0800, Zhang Qiao wrote:
> I checked the code again.
> I don't quite understand what you said, if the child be moved between
> cgroup_post_fork() and sched_post_sched(), what problems might it cause?

cgroup_post_fork() is where the child's creation is committed from cgroup's
POV, so it'd be migrating cgroups before the initial creation is finished.
From glancing, looks like it'll break css_set task counts to begin with.
This violates the basic assumptions and can cause critical failures in
subtle ways. The would replace one subtle race with a possibly worse one.

Thanks.

-- 
tejun

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Zhang Qiao]

On 2021/9/8 1:01, Tejun Heo wrote:
> Hello,
> 
> On Thu, Sep 02, 2021 at 03:42:15PM +0800, Zhang Qiao wrote:
>> I checked the code again.
>> I don't quite understand what you said, if the child be moved between
>> cgroup_post_fork() and sched_post_sched(), what problems might it cause?
> 
> cgroup_post_fork() is where the child's creation is committed from cgroup's
> POV, so it'd be migrating cgroups before the initial creation is finished.
>>From glancing, looks like it'll break css_set task counts to begin with.
> This violates the basic assumptions and can cause critical failures in
> subtle ways. The would replace one subtle race with a possibly worse one.

Hello,

I will update this patch by following the steps below:
1)rename cgroup_subsys->fork() to cgroup_subsys->post_fork();
2)add cgroup_subsys->fork() and the cpu_cgroup_fork() callback like this:

void cpu_cgroup_fork(struct task_struct *task) {
....
	p->sched_task_group = task_group(current);
	__set_task_cpu(p, smp_processor_id());
	if (p->sched_class->task_fork)
		p->sched_class->task_fork(p);
....


3)call cgroup_subsys->fork() after cgroup_can_fork().

Do you have any suggestion?
thanks.

Zhang Qiao

> 
> Thanks.
> 

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Tejun Heo]

Hello,

On Wed, Sep 08, 2021 at 07:32:06PM +0800, Zhang Qiao wrote:
> I will update this patch by following the steps below:
> 1)rename cgroup_subsys->fork() to cgroup_subsys->post_fork();
> 2)add cgroup_subsys->fork() and the cpu_cgroup_fork() callback like this:
> 
> void cpu_cgroup_fork(struct task_struct *task) {
> ....
> 	p->sched_task_group = task_group(current);
> 	__set_task_cpu(p, smp_processor_id());
> 	if (p->sched_class->task_fork)
> 		p->sched_class->task_fork(p);
> ....
> 
> 
> 3)call cgroup_subsys->fork() after cgroup_can_fork().
> 
> Do you have any suggestion?

I'm not following why it needs to shuffle the callbacks. Can't you just
relocate the fectching of task_group after can_fork?

Thanks.

-- 
tejun

Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Zhang Qiao]

在 2021/9/9 0:29, Tejun Heo 写道:
> Hello,
> 
> On Wed, Sep 08, 2021 at 07:32:06PM +0800, Zhang Qiao wrote:
>> I will update this patch by following the steps below:
>> 1)rename cgroup_subsys->fork() to cgroup_subsys->post_fork();
>> 2)add cgroup_subsys->fork() and the cpu_cgroup_fork() callback like this:
>>
>> void cpu_cgroup_fork(struct task_struct *task) {
>> ....
>> 	p->sched_task_group = task_group(current);
>> 	__set_task_cpu(p, smp_processor_id());
>> 	if (p->sched_class->task_fork)
>> 		p->sched_class->task_fork(p);
>> ....
>>
>>
>> 3)call cgroup_subsys->fork() after cgroup_can_fork().
>>
>> Do you have any suggestion?
> 
> I'm not following why it needs to shuffle the callbacks. Can't you just
Just to provide a placeholder to call task_fork(). But thought about it,
there are indeed some problems with this modification.

Thanks.

Zhang Qiao

> relocate the fectching of task_group after can_fork?>
> Thanks.
>