[syzbot] WARNING in kthread_is_per_cpu

[syzbot] WARNING in kthread_is_per_cpu

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    1216f02e Add linux-next specific files for 20210415
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1032ba29d00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=3491b04113499f81
dashboard link: https://syzkaller.appspot.com/bug?extid=9362b31a2e0cad8b749d

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+9362b31a2e0cad8b749d@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: CPU: 1 PID: 23550 at kernel/kthread.c:83 to_kthread kernel/kthread.c:83 [inline]
WARNING: CPU: 1 PID: 23550 at kernel/kthread.c:83 kthread_is_per_cpu+0xc4/0xf0 kernel/kthread.c:519
Modules linked in:
CPU: 1 PID: 23550 Comm: syz-executor.3 Not tainted 5.12.0-rc7-next-20210415-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:to_kthread kernel/kthread.c:83 [inline]
RIP: 0010:kthread_is_per_cpu+0xc4/0xf0 kernel/kthread.c:519
Code: 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 75 2e 4c 8b 23 41 83 e4 01 e8 89 d3 27 00 44 89 e0 5b 5d 41 5c c3 e8 7c d3 27 00 <0f> 0b eb 88 e8 33 90 6c 00 e9 68 ff ff ff e8 39 90 6c 00 eb 9a 48
RSP: 0018:ffffc90000dc0c08 EFLAGS: 00010046
RAX: 0000000000000000 RBX: ffff88802533d580 RCX: 0000000000000100
RDX: ffff8880549bb900 RSI: ffffffff814ca4c4 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000000 R09: ffff88802533d580
R10: ffffffff814ca44c R11: 00000000018a3b90 R12: 0000000000000001
R13: ffffc90000dc0d90 R14: 0000000000000001 R15: ffff88802533d580
FS:  00007f4be57d3700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b2cd24000 CR3: 0000000024626000 CR4: 00000000001506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <IRQ>
 can_migrate_task+0x124/0x1630 kernel/sched/fair.c:7610
 detach_tasks kernel/sched/fair.c:7774 [inline]
 load_balance+0xc72/0x2730 kernel/sched/fair.c:9696
 rebalance_domains+0x668/0xda0 kernel/sched/fair.c:10075
 __do_softirq+0x29b/0x9fe kernel/softirq.c:559
 invoke_softirq kernel/softirq.c:433 [inline]
 __irq_exit_rcu+0x136/0x200 kernel/softirq.c:637
 irq_exit_rcu+0x5/0x20 kernel/softirq.c:649
 sysvec_apic_timer_interrupt+0x93/0xc0 arch/x86/kernel/apic/apic.c:1100
 </IRQ>
 asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:632
RIP: 0010:__raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:161 [inline]
RIP: 0010:_raw_spin_unlock_irqrestore+0x38/0x70 kernel/locking/spinlock.c:191
Code: 74 24 10 e8 5a 05 46 f8 48 89 ef e8 f2 7d 46 f8 81 e3 00 02 00 00 75 25 9c 58 f6 c4 02 75 2d 48 85 db 74 01 fb bf 01 00 00 00 <e8> e3 83 3a f8 65 8b 05 cc e4 ed 76 85 c0 74 0a 5b 5d c3 e8 20 73
RSP: 0018:ffffc90001c0ef30 EFLAGS: 00000206
RAX: 0000000000000006 RBX: 0000000000000200 RCX: 1ffffffff1fbdad2
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000001
RBP: ffffffff8c076c20 R08: 0000000000000001 R09: ffffffff8fdeb8a7
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
R13: 000000000000000c R14: 0000000000000002 R15: 000000000008f9b0
 spin_unlock_irqrestore include/linux/spinlock.h:414 [inline]
 pcpu_alloc+0x4f7/0x17a0 mm/percpu.c:1807
 vlan_dev_init+0x9f9/0xe70 net/8021q/vlan_dev.c:614
 register_netdevice+0x51e/0x1500 net/core/dev.c:10188
 register_vlan_dev+0x360/0x960 net/8021q/vlan.c:179
 vlan_newlink+0x477/0x700 net/8021q/vlan_netlink.c:187
 __rtnl_newlink+0x1062/0x1710 net/core/rtnetlink.c:3452
 rtnl_newlink+0x64/0xa0 net/core/rtnetlink.c:3500
 rtnetlink_rcv_msg+0x413/0xaf0 net/core/rtnetlink.c:5562
 netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2502
 netlink_unicast_kernel net/netlink/af_netlink.c:1312 [inline]
 netlink_unicast+0x533/0x7d0 net/netlink/af_netlink.c:1338
 netlink_sendmsg+0x84c/0xd90 net/netlink/af_netlink.c:1927
 sock_sendmsg_nosec net/socket.c:654 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:674
 ____sys_sendmsg+0x6e8/0x810 net/socket.c:2350
 ___sys_sendmsg+0xf3/0x170 net/socket.c:2404
 __sys_sendmsg+0xe5/0x1b0 net/socket.c:2433
 do_syscall_64+0x3a/0xb0 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x466459
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f4be57d3188 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 000000000056bf60 RCX: 0000000000466459
RDX: 0000000000000810 RSI: 0000000020000000 RDI: 0000000000000006
RBP: 00000000004bf9fb R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000056bf60
R13: 00007ffc92da70af R14: 00007f4be57d3300 R15: 0000000000022000


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

Re: [syzbot] WARNING in kthread_is_per_cpu

[Thomas Gleixner]

On Mon, Apr 19 2021 at 03:36, syzbot wrote:

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    1216f02e Add linux-next specific files for 20210415
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1032ba29d00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=3491b04113499f81
> dashboard link: https://syzkaller.appspot.com/bug?extid=9362b31a2e0cad8b749d
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+9362b31a2e0cad8b749d@syzkaller.appspotmail.com
>
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 23550 at kernel/kthread.c:83 to_kthread kernel/kthread.c:83 [inline]
> WARNING: CPU: 1 PID: 23550 at kernel/kthread.c:83 kthread_is_per_cpu+0xc4/0xf0 kernel/kthread.c:519
> Modules linked in:
> CPU: 1 PID: 23550 Comm: syz-executor.3 Not tainted 5.12.0-rc7-next-20210415-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:to_kthread kernel/kthread.c:83 [inline]
> RIP: 0010:kthread_is_per_cpu+0xc4/0xf0 kernel/kthread.c:519
> Code: 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 75 2e 4c 8b 23 41 83 e4 01 e8 89 d3 27 00 44 89 e0 5b 5d 41 5c c3 e8 7c d3 27 00 <0f> 0b eb 88 e8 33 90 6c 00 e9 68 ff ff ff e8 39 90 6c 00 eb 9a 48
> RSP: 0018:ffffc90000dc0c08 EFLAGS: 00010046
> RAX: 0000000000000000 RBX: ffff88802533d580 RCX: 0000000000000100
> RDX: ffff8880549bb900 RSI: ffffffff814ca4c4 RDI: 0000000000000003
> RBP: 0000000000000000 R08: 0000000000000000 R09: ffff88802533d580
> R10: ffffffff814ca44c R11: 00000000018a3b90 R12: 0000000000000001
> R13: ffffc90000dc0d90 R14: 0000000000000001 R15: ffff88802533d580
> FS:  00007f4be57d3700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000001b2cd24000 CR3: 0000000024626000 CR4: 00000000001506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <IRQ>
>  can_migrate_task+0x124/0x1630 kernel/sched/fair.c:7610

So this is:

if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))

The warning in to_kthread() is:

    WARN_ON(!(k->flags & PF_KTHREAD));

IOW, the p>flags lost PF_KTHREAD within at max. 50 instructions.

Magic, cosmic rays or memory corruption / stray pointer in some other
place?

Thanks,

        tglx

Re: [syzbot] WARNING in kthread_is_per_cpu

[Valentin Schneider]

On 19/04/21 03:36, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    1216f02e Add linux-next specific files for 20210415
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1032ba29d00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=3491b04113499f81
> dashboard link: https://syzkaller.appspot.com/bug?extid=9362b31a2e0cad8b749d
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+9362b31a2e0cad8b749d@syzkaller.appspotmail.com
>
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 23550 at kernel/kthread.c:83 to_kthread kernel/kthread.c:83 [inline]
> WARNING: CPU: 1 PID: 23550 at kernel/kthread.c:83 kthread_is_per_cpu+0xc4/0xf0 kernel/kthread.c:519
> Modules linked in:
> CPU: 1 PID: 23550 Comm: syz-executor.3 Not tainted 5.12.0-rc7-next-20210415-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:to_kthread kernel/kthread.c:83 [inline]
> RIP: 0010:kthread_is_per_cpu+0xc4/0xf0 kernel/kthread.c:519
> Code: 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 75 2e 4c 8b 23 41 83 e4 01 e8 89 d3 27 00 44 89 e0 5b 5d 41 5c c3 e8 7c d3 27 00 <0f> 0b eb 88 e8 33 90 6c 00 e9 68 ff ff ff e8 39 90 6c 00 eb 9a 48
> RSP: 0018:ffffc90000dc0c08 EFLAGS: 00010046
> RAX: 0000000000000000 RBX: ffff88802533d580 RCX: 0000000000000100
> RDX: ffff8880549bb900 RSI: ffffffff814ca4c4 RDI: 0000000000000003
> RBP: 0000000000000000 R08: 0000000000000000 R09: ffff88802533d580
> R10: ffffffff814ca44c R11: 00000000018a3b90 R12: 0000000000000001
> R13: ffffc90000dc0d90 R14: 0000000000000001 R15: ffff88802533d580
> FS:  00007f4be57d3700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000001b2cd24000 CR3: 0000000024626000 CR4: 00000000001506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <IRQ>
>  can_migrate_task+0x124/0x1630 kernel/sched/fair.c:7610
>  detach_tasks kernel/sched/fair.c:7774 [inline]
>  load_balance+0xc72/0x2730 kernel/sched/fair.c:9696
>  rebalance_domains+0x668/0xda0 kernel/sched/fair.c:10075
>  __do_softirq+0x29b/0x9fe kernel/softirq.c:559
>  invoke_softirq kernel/softirq.c:433 [inline]
>  __irq_exit_rcu+0x136/0x200 kernel/softirq.c:637
>  irq_exit_rcu+0x5/0x20 kernel/softirq.c:649
>  sysvec_apic_timer_interrupt+0x93/0xc0 arch/x86/kernel/apic/apic.c:1100
>  </IRQ>
>  asm_sysvec_apic_timer_interrupt+0x12/0x20
> arch/x86/include/asm/idtentry.h:632


  if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
                                 `\
                                   to_kthread(p);
                                    `\
                                      WARN_ON(!(p->flags & PF_KTHREAD));

... Huh?

Re: [syzbot] WARNING in kthread_is_per_cpu

[Peter Zijlstra]

On Mon, Apr 19, 2021 at 12:31:22PM +0100, Valentin Schneider wrote:

>   if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
>                                  `\
>                                    to_kthread(p);
>                                     `\
>                                       WARN_ON(!(p->flags & PF_KTHREAD));
> 
> ... Huh?

Something like so perhaps?

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 1578973c5740..eeba40df61ac 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -78,6 +78,14 @@ static inline void set_kthread_struct(void *kthread)
 	current->set_child_tid = (__force void __user *)kthread;
 }
 
+static inline struct kthread *__to_kthread(struct task_struct *k)
+{
+	void *kthread = (__force void *)k->set_child_tid;
+	if (kthread && !(k->flags & PF_KTHREAD))
+		kthread = NULL;
+	return kthread;
+}
+
 static inline struct kthread *to_kthread(struct task_struct *k)
 {
 	WARN_ON(!(k->flags & PF_KTHREAD));
@@ -516,7 +524,7 @@ void kthread_set_per_cpu(struct task_struct *k, int cpu)
 
 bool kthread_is_per_cpu(struct task_struct *k)
 {
-	struct kthread *kthread = to_kthread(k);
+	struct kthread *kthread = __to_kthread(k);
 	if (!kthread)
 		return false;
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3384ea74cad4..dc6311bd6986 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7658,7 +7658,7 @@ static void balance_push(struct rq *rq)
 	 * histerical raisins.
 	 */
 	if (rq->idle == push_task ||
-	    ((push_task->flags & PF_KTHREAD) && kthread_is_per_cpu(push_task)) ||
+	    kthread_is_per_cpu(push_task) ||
 	    is_migration_disabled(push_task)) {
 
 		/*

Re: [syzbot] WARNING in kthread_is_per_cpu

[Valentin Schneider]

On 19/04/21 20:45, Peter Zijlstra wrote:
> On Mon, Apr 19, 2021 at 12:31:22PM +0100, Valentin Schneider wrote:
>
>>   if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
>>                                  `\
>>                                    to_kthread(p);
>>                                     `\
>>                                       WARN_ON(!(p->flags & PF_KTHREAD));
>>
>> ... Huh?
>
> Something like so perhaps?
>

Looks about right, IIUC the key being:

  p->flags & PF_KTHREAD + p->set_child_tid => the struct kthread is
  persistent

  p->flags & PF_KTHREAD => you may or may not have a struct kthread (see
  kernel/umh.c kernel_thread() uses). PF_KTHREAD isn't even guaranteed to
  persist (begin_new_exec()), which seems to be what the syzbot hit.

I'd be happy to see is_per_cpu_kthread() die, but that's somewhat
orthogonal to this here. For now, this does need the tiny extra below.

While we're at it, does free_kthread_struct() want the __to_kthread()
treatment as well? The other to_kthread() callsites looked like they only
made sense with a "proper" kthread anyway.

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 49636a49843f..8b470c2d5680 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7612,7 +7612,7 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 		return 0;
 
 	/* Disregard pcpu kthreads; they are where they need to be. */
-	if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
+	if (kthread_is_per_cpu(p))
 		return 0;
 
 	if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {

> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 1578973c5740..eeba40df61ac 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -78,6 +78,14 @@ static inline void set_kthread_struct(void *kthread)
>       current->set_child_tid = (__force void __user *)kthread;
>  }
>
> +static inline struct kthread *__to_kthread(struct task_struct *k)
> +{
> +	void *kthread = (__force void *)k->set_child_tid;
> +	if (kthread && !(k->flags & PF_KTHREAD))
> +		kthread = NULL;
> +	return kthread;
> +}
> +
>  static inline struct kthread *to_kthread(struct task_struct *k)
>  {
>       WARN_ON(!(k->flags & PF_KTHREAD));
> @@ -516,7 +524,7 @@ void kthread_set_per_cpu(struct task_struct *k, int cpu)
>
>  bool kthread_is_per_cpu(struct task_struct *k)
>  {
> -	struct kthread *kthread = to_kthread(k);
> +	struct kthread *kthread = __to_kthread(k);
>       if (!kthread)
>               return false;
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3384ea74cad4..dc6311bd6986 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7658,7 +7658,7 @@ static void balance_push(struct rq *rq)
>        * histerical raisins.
>        */
>       if (rq->idle == push_task ||
> -	    ((push_task->flags & PF_KTHREAD) && kthread_is_per_cpu(push_task)) ||
> +	    kthread_is_per_cpu(push_task) ||
>           is_migration_disabled(push_task)) {
>
>               /*

Re: [syzbot] WARNING in kthread_is_per_cpu

[Peter Zijlstra]

On Mon, Apr 19, 2021 at 08:58:26PM +0100, Valentin Schneider wrote:

> Looks about right, IIUC the key being:
> 
>   p->flags & PF_KTHREAD + p->set_child_tid => the struct kthread is
>   persistent
> 
>   p->flags & PF_KTHREAD => you may or may not have a struct kthread (see
>   kernel/umh.c kernel_thread() uses). PF_KTHREAD isn't even guaranteed to
>   persist (begin_new_exec()), which seems to be what the syzbot hit.

Ack, that's nicely put.

> While we're at it, does free_kthread_struct() want the __to_kthread()
> treatment as well? The other to_kthread() callsites looked like they only
> made sense with a "proper" kthread anyway.

I think free_kthread_struct() is ok, because a task at that point in its
lifetime cannot be also doing exec().

kthread_func() is another 'fun' trainwreck waiting to happen -- luckily
the only caller uses current, still let me go fix it.

kthread_probe_data() relies on PF_WQ_WORKER implying PF_KTHREAD but
otherwise seems very fragile too.

Something like so then?

---
Subject: kthread: Fix PF_KTHREAD vs to_kthread() race
From: Peter Zijlstra <peterz@infradead.org>
Date: Tue Apr 20 10:18:17 CEST 2021

The kthread_is_per_cpu() construct relies on only being called on
PF_KTHREAD tasks (per the WARN in to_kthread). This gives rise to the
following usage pattern:

	if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))

However, as reported by syzcaller, this is broken. The scenario is:

	CPU0				CPU1 (running p)

	(p->flags & PF_KTHREAD) // true

					begin_new_exec()
					  me->flags &= ~(PF_KTHREAD|...);
	kthread_is_per_cpu(p)
	  to_kthread(p)
	    WARN(!(p->flags & PF_KTHREAD) <-- *SPLAT*

Introduce __to_kthread() that omits the WARN and is sure to check both
values.

Use this to remove the problematic pattern for kthread_is_per_cpu()
and fix a number of other kthread_*() functions that have similar
issues but are currently not used in ways that would expose the
problem.

Notably kthread_func() is only ever called on 'current', while
kthread_probe_data() is only used for PF_WQ_WORKER, which implies the
task is from kthread_create*().

Fixes: ac687e6e8c26 ("kthread: Extract KTHREAD_IS_PER_CPU")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/kthread.c    |   33 +++++++++++++++++++++++++++------
 kernel/sched/core.c |    2 +-
 kernel/sched/fair.c |    2 +-
 3 files changed, 29 insertions(+), 8 deletions(-)

--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -84,6 +84,25 @@ static inline struct kthread *to_kthread
 	return (__force void *)k->set_child_tid;
 }
 
+/*
+ * Variant of to_kthread() that doesn't assume @p is a kthread.
+ *
+ * Per construction; when:
+ *
+ *   (p->flags & PF_KTHREAD) && p->set_child_tid
+ *
+ * the task is both a kthread and struct kthread is persistent. However
+ * PF_KTHREAD on it's own is not, kernel_thread() can exec() (See umh.c and
+ * begin_new_exec()).
+ */
+static inline struct kthread *__to_kthread(struct task_struct *p)
+{
+	void *kthread = (__force void *)p->set_child_tid;
+	if (kthread && !(p->flags & PF_KTHREAD))
+		kthread = NULL;
+	return kthread;
+}
+
 void free_kthread_struct(struct task_struct *k)
 {
 	struct kthread *kthread;
@@ -168,8 +187,9 @@ EXPORT_SYMBOL_GPL(kthread_freezable_shou
  */
 void *kthread_func(struct task_struct *task)
 {
-	if (task->flags & PF_KTHREAD)
-		return to_kthread(task)->threadfn;
+	struct kthread *kthread = __to_kthread(task);
+	if (kthread)
+		return kthread->threadfn;
 	return NULL;
 }
 EXPORT_SYMBOL_GPL(kthread_func);
@@ -199,10 +219,11 @@ EXPORT_SYMBOL_GPL(kthread_data);
  */
 void *kthread_probe_data(struct task_struct *task)
 {
-	struct kthread *kthread = to_kthread(task);
+	struct kthread *kthread = __to_kthread(task);
 	void *data = NULL;
 
-	copy_from_kernel_nofault(&data, &kthread->data, sizeof(data));
+	if (kthread)
+		copy_from_kernel_nofault(&data, &kthread->data, sizeof(data));
 	return data;
 }
 
@@ -514,9 +535,9 @@ void kthread_set_per_cpu(struct task_str
 	set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
 }
 
-bool kthread_is_per_cpu(struct task_struct *k)
+bool kthread_is_per_cpu(struct task_struct *p)
 {
-	struct kthread *kthread = to_kthread(k);
+	struct kthread *kthread = __to_kthread(p);
 	if (!kthread)
 		return false;
 
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8505,7 +8505,7 @@ static void balance_push(struct rq *rq)
 	 * histerical raisins.
 	 */
 	if (rq->idle == push_task ||
-	    ((push_task->flags & PF_KTHREAD) && kthread_is_per_cpu(push_task)) ||
+	    kthread_is_per_cpu(push_task) ||
 	    is_migration_disabled(push_task)) {
 
 		/*
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7619,7 +7619,7 @@ int can_migrate_task(struct task_struct
 		return 0;
 
 	/* Disregard pcpu kthreads; they are where they need to be. */
-	if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
+	if (kthread_is_per_cpu(p))
 		return 0;
 
 	if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {

Re: [syzbot] WARNING in kthread_is_per_cpu

[Valentin Schneider]

On 20/04/21 10:51, Peter Zijlstra wrote:
> On Mon, Apr 19, 2021 at 08:58:26PM +0100, Valentin Schneider wrote:
>
>> Looks about right, IIUC the key being:
>>
>>   p->flags & PF_KTHREAD + p->set_child_tid => the struct kthread is
>>   persistent
>>
>>   p->flags & PF_KTHREAD => you may or may not have a struct kthread (see
>>   kernel/umh.c kernel_thread() uses). PF_KTHREAD isn't even guaranteed to
>>   persist (begin_new_exec()), which seems to be what the syzbot hit.
>
> Ack, that's nicely put.
>
>> While we're at it, does free_kthread_struct() want the __to_kthread()
>> treatment as well? The other to_kthread() callsites looked like they only
>> made sense with a "proper" kthread anyway.
>
> I think free_kthread_struct() is ok, because a task at that point in its
> lifetime cannot be also doing exec().
>

What if it's one of those kthreads created by directly invoking
kernel_thread()? AFAICT right now it's only umh, and that one does execve()
so it ends up stripped of PF_KTHREAD. It could however go through an error
path, i.e. not call exec, and exit, giving us:

  put_task_struct(p)
  `\
    free_task(p)
    `\
      if (tsk->flags & PF_KTHREAD)
          free_kthread_struct(tsk);
          `\
            to_kthread(p)

> kthread_func() is another 'fun' trainwreck waiting to happen -- luckily
> the only caller uses current, still let me go fix it.
>
> kthread_probe_data() relies on PF_WQ_WORKER implying PF_KTHREAD but
> otherwise seems very fragile too.
>
> Something like so then?
>

Other than the above:

Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com>

Re: [syzbot] WARNING in kthread_is_per_cpu

[Peter Zijlstra]

On Tue, Apr 20, 2021 at 10:43:43AM +0100, Valentin Schneider wrote:
> On 20/04/21 10:51, Peter Zijlstra wrote:

> > I think free_kthread_struct() is ok, because a task at that point in its
> > lifetime cannot be also doing exec().
> >
> 
> What if it's one of those kthreads created by directly invoking
> kernel_thread()? AFAICT right now it's only umh, and that one does execve()
> so it ends up stripped of PF_KTHREAD. It could however go through an error
> path, i.e. not call exec, and exit, giving us:
> 
>   put_task_struct(p)
>   `\
>     free_task(p)
>     `\
>       if (tsk->flags & PF_KTHREAD)
>           free_kthread_struct(tsk);
>           `\
>             to_kthread(p)

I'm not following, at the point we hit free_task() it had better be dead
and p->flags had better be stable. Either it will, or will not, have
PF_KTHREAD.

Re: [syzbot] WARNING in kthread_is_per_cpu

[Valentin Schneider]

On 20/04/21 12:11, Peter Zijlstra wrote:
> On Tue, Apr 20, 2021 at 10:43:43AM +0100, Valentin Schneider wrote:
>> On 20/04/21 10:51, Peter Zijlstra wrote:
>
>> > I think free_kthread_struct() is ok, because a task at that point in its
>> > lifetime cannot be also doing exec().
>> >
>>
>> What if it's one of those kthreads created by directly invoking
>> kernel_thread()? AFAICT right now it's only umh, and that one does execve()
>> so it ends up stripped of PF_KTHREAD. It could however go through an error
>> path, i.e. not call exec, and exit, giving us:
>>
>>   put_task_struct(p)
>>   `\
>>     free_task(p)
>>     `\
>>       if (tsk->flags & PF_KTHREAD)
>>           free_kthread_struct(tsk);
>>           `\
>>             to_kthread(p)
>
> I'm not following, at the point we hit free_task() it had better be dead
> and p->flags had better be stable. Either it will, or will not, have
> PF_KTHREAD.

Bah, don't mind me, for some reason I was obsessed by that umh thing of
having

  (p->flags & PF_KTHREAD) && !p->set_child_tid

but that's not a problem there. Sorry about that.

[tip: sched/core] kthread: Fix PF_KTHREAD vs to_kthread() race

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     3a7956e25e1d7b3c148569e78895e1f3178122a9
Gitweb:        https://git.kernel.org/tip/3a7956e25e1d7b3c148569e78895e1f3178122a9
Author:        Peter Zijlstra <peterz@infradead.org>
AuthorDate:    Tue, 20 Apr 2021 10:18:17 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 21 Apr 2021 13:55:42 +02:00

kthread: Fix PF_KTHREAD vs to_kthread() race

The kthread_is_per_cpu() construct relies on only being called on
PF_KTHREAD tasks (per the WARN in to_kthread). This gives rise to the
following usage pattern:

	if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))

However, as reported by syzcaller, this is broken. The scenario is:

	CPU0				CPU1 (running p)

	(p->flags & PF_KTHREAD) // true

					begin_new_exec()
					  me->flags &= ~(PF_KTHREAD|...);
	kthread_is_per_cpu(p)
	  to_kthread(p)
	    WARN(!(p->flags & PF_KTHREAD) <-- *SPLAT*

Introduce __to_kthread() that omits the WARN and is sure to check both
values.

Use this to remove the problematic pattern for kthread_is_per_cpu()
and fix a number of other kthread_*() functions that have similar
issues but are currently not used in ways that would expose the
problem.

Notably kthread_func() is only ever called on 'current', while
kthread_probe_data() is only used for PF_WQ_WORKER, which implies the
task is from kthread_create*().

Fixes: ac687e6e8c26 ("kthread: Extract KTHREAD_IS_PER_CPU")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com>
Link: https://lkml.kernel.org/r/YH6WJc825C4P0FCK@hirez.programming.kicks-ass.net
---
 kernel/kthread.c    | 33 +++++++++++++++++++++++++++------
 kernel/sched/core.c |  2 +-
 kernel/sched/fair.c |  2 +-
 3 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 1578973..6d3c488 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -84,6 +84,25 @@ static inline struct kthread *to_kthread(struct task_struct *k)
 	return (__force void *)k->set_child_tid;
 }
 
+/*
+ * Variant of to_kthread() that doesn't assume @p is a kthread.
+ *
+ * Per construction; when:
+ *
+ *   (p->flags & PF_KTHREAD) && p->set_child_tid
+ *
+ * the task is both a kthread and struct kthread is persistent. However
+ * PF_KTHREAD on it's own is not, kernel_thread() can exec() (See umh.c and
+ * begin_new_exec()).
+ */
+static inline struct kthread *__to_kthread(struct task_struct *p)
+{
+	void *kthread = (__force void *)p->set_child_tid;
+	if (kthread && !(p->flags & PF_KTHREAD))
+		kthread = NULL;
+	return kthread;
+}
+
 void free_kthread_struct(struct task_struct *k)
 {
 	struct kthread *kthread;
@@ -168,8 +187,9 @@ EXPORT_SYMBOL_GPL(kthread_freezable_should_stop);
  */
 void *kthread_func(struct task_struct *task)
 {
-	if (task->flags & PF_KTHREAD)
-		return to_kthread(task)->threadfn;
+	struct kthread *kthread = __to_kthread(task);
+	if (kthread)
+		return kthread->threadfn;
 	return NULL;
 }
 EXPORT_SYMBOL_GPL(kthread_func);
@@ -199,10 +219,11 @@ EXPORT_SYMBOL_GPL(kthread_data);
  */
 void *kthread_probe_data(struct task_struct *task)
 {
-	struct kthread *kthread = to_kthread(task);
+	struct kthread *kthread = __to_kthread(task);
 	void *data = NULL;
 
-	copy_from_kernel_nofault(&data, &kthread->data, sizeof(data));
+	if (kthread)
+		copy_from_kernel_nofault(&data, &kthread->data, sizeof(data));
 	return data;
 }
 
@@ -514,9 +535,9 @@ void kthread_set_per_cpu(struct task_struct *k, int cpu)
 	set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
 }
 
-bool kthread_is_per_cpu(struct task_struct *k)
+bool kthread_is_per_cpu(struct task_struct *p)
 {
-	struct kthread *kthread = to_kthread(k);
+	struct kthread *kthread = __to_kthread(p);
 	if (!kthread)
 		return false;
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fcb35ae..4a0668a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7667,7 +7667,7 @@ static void balance_push(struct rq *rq)
 	 * histerical raisins.
 	 */
 	if (rq->idle == push_task ||
-	    ((push_task->flags & PF_KTHREAD) && kthread_is_per_cpu(push_task)) ||
+	    kthread_is_per_cpu(push_task) ||
 	    is_migration_disabled(push_task)) {
 
 		/*
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7ea3b93..1d75af1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7612,7 +7612,7 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 		return 0;
 
 	/* Disregard pcpu kthreads; they are where they need to be. */
-	if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
+	if (kthread_is_per_cpu(p))
 		return 0;
 
 	if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {