Re: [syzbot] [kernel?] WARNING in flush_cpu_slab

Re: [syzbot] [kernel?] WARNING in flush_cpu_slab

[Thomas Gleixner]

On Thu, May 23 2024 at 23:03, Vlastimil Babka wrote:
> On 5/23/24 12:36 PM, Thomas Gleixner wrote:
>>> ------------[ cut here ]------------
>>> DEBUG_LOCKS_WARN_ON(l->owner)
>>> WARNING: CPU: 3 PID: 5221 at include/linux/local_lock_internal.h:30 local_lock_acquire include/linux/local_lock_internal.h:30 [inline]
>>> WARNING: CPU: 3 PID: 5221 at include/linux/local_lock_internal.h:30 flush_slab mm/slub.c:3088 [inline]
>>> WARNING: CPU: 3 PID: 5221 at include/linux/local_lock_internal.h:30 flush_cpu_slab+0x37f/0x410 mm/slub.c:3146
>
> I'm puzzled by this. We use local_lock_irqsave() on !PREEMPT_RT everywhere.
> IIUC this warning says we did the irqsave() and then found out somebody else
> already set the owner? But that means they also did that irqsave() and set
> themselves as l->owner. Does that mey there would be a spurious irq enable
> that didn't go through local_unlock_irqrestore()?
>
> Also this particular stack is from the work, which is scheduled by
> queue_work_on() in flush_all_cpus_locked(), which also has a
> lockdep_assert_cpus_held() so it should fullfill the "the caller must ensure
> the cpu doesn't go away" property. But I think even if this ended up on the
> wrong cpu (for the full duration or migrated while processing the work item)
> somehow, it wouldn't be able to cause such warning, but rather corrupt
> something else

Indeed. There is another report which makes no sense either:

 https://lore.kernel.org/lkml/000000000000fa09d906191c3ee5@google.com

Both look like data corropution issues caused by whatever...

Thanks,

        tglx

Re: [syzbot] [kernel?] WARNING in flush_cpu_slab

[Vlastimil Babka]

On 5/24/24 12:32 AM, Thomas Gleixner wrote:
> On Thu, May 23 2024 at 23:03, Vlastimil Babka wrote:
>> On 5/23/24 12:36 PM, Thomas Gleixner wrote:
>>>> ------------[ cut here ]------------
>>>> DEBUG_LOCKS_WARN_ON(l->owner)
>>>> WARNING: CPU: 3 PID: 5221 at include/linux/local_lock_internal.h:30 local_lock_acquire include/linux/local_lock_internal.h:30 [inline]
>>>> WARNING: CPU: 3 PID: 5221 at include/linux/local_lock_internal.h:30 flush_slab mm/slub.c:3088 [inline]
>>>> WARNING: CPU: 3 PID: 5221 at include/linux/local_lock_internal.h:30 flush_cpu_slab+0x37f/0x410 mm/slub.c:3146
>>
>> I'm puzzled by this. We use local_lock_irqsave() on !PREEMPT_RT everywhere.
>> IIUC this warning says we did the irqsave() and then found out somebody else
>> already set the owner? But that means they also did that irqsave() and set
>> themselves as l->owner. Does that mey there would be a spurious irq enable
>> that didn't go through local_unlock_irqrestore()?
>>
>> Also this particular stack is from the work, which is scheduled by
>> queue_work_on() in flush_all_cpus_locked(), which also has a
>> lockdep_assert_cpus_held() so it should fullfill the "the caller must ensure
>> the cpu doesn't go away" property. But I think even if this ended up on the
>> wrong cpu (for the full duration or migrated while processing the work item)
>> somehow, it wouldn't be able to cause such warning, but rather corrupt
>> something else
> 
> Indeed. There is another report which makes no sense either:
> 
>  https://lore.kernel.org/lkml/000000000000fa09d906191c3ee5@google.com

That looks like slab->next which should contain a valid pointer or NULL,
contains 0x13.
slab->next is initialized in put_cpu_partial() from s->cpu_slab->partial

Here we have corruption inside s->cpu_slab->list_lock

> Both look like data corropution issues caused by whatever...

s->cpu_slab is percpu allocation so possibly another percpu alloc user has a
buffer overflow?

> Thanks,
> 
>         tglx

Re: [syzbot] [kernel?] WARNING in flush_cpu_slab

[Sebastian Andrzej Siewior]

On 2024-05-23 23:03:52 [+0200], Vlastimil Babka wrote:
> I'm puzzled by this. We use local_lock_irqsave() on !PREEMPT_RT everywhere.
> IIUC this warning says we did the irqsave() and then found out somebody else
> already set the owner? But that means they also did that irqsave() and set
> themselves as l->owner. Does that mey there would be a spurious irq enable
> that didn't go through local_unlock_irqrestore()?

correct.

> 
> Also this particular stack is from the work, which is scheduled by
> queue_work_on() in flush_all_cpus_locked(), which also has a
> lockdep_assert_cpus_held() so it should fullfill the "the caller must ensure
> the cpu doesn't go away" property. But I think even if this ended up on the
> wrong cpu (for the full duration or migrated while processing the work item)
> somehow, it wouldn't be able to cause such warning, but rather corrupt
> something else

Based on

> >> CPU: 3 PID: 5221 Comm: kworker/3:3 Not tainted 6.9.0-syzkaller-10713-g2a8120d7b482 #0

the code was invoked on CPU3 and the kworker was made for CPU3. This is
all fine. All access for the lock in question is within a few lines so
there is no unbalance lock/ unlock or IRQ-unlock which could explain it.

Sebastian