linux-kernel - Re: WARNING in __mutex_unlock

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Date:   Tue, 8 May 2018 06:49:47 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        syzbot <syzbot+35666cba7f0a337e2e79@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com
Cc:     hpa@...or.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, rkrcmar@...hat.com, tglx@...utronix.de,
        x86@...nel.org, Roman Kagan <rkagan@...tuozzo.com>,
        Cathy Avery <cavery@...hat.com>
Subject: Re: WARNING in __mutex_unlock_slowpath

On 2018/05/08 2:19, Paolo Bonzini wrote:
>> ------------[ cut here ]------------
>> DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current)
>> WARNING: CPU: 0 PID: 4525 at kernel/locking/mutex.c:1032
>> __mutex_unlock_slowpath+0x62e/0x8a0 kernel/locking/mutex.c:1032
>> Kernel panic - not syncing: panic_on_warn set ...
> 
> This doesn't make much sense, unless it's a "generic" memory corruption,
> but at least the reproducer seems to be simple, just (in pseudocode)
> 
>   ioctl(kvm_vm_fd, KVM_HYPERV_EVENTFD,
>         { fd = some_eventfd, conn_id = 0, flags = 0 })
>   ioctl(kvm_vm_fd, KVM_HYPERV_EVENTFD,
>         { fd = -1, conn_id = 5, flags = KVM_HYPERV_EVENTFD_DEASSIGN })
> 

This makes much sense if this is use-after-free memory access which was
manifested differently due to reallocated after released.

  mutex_lock(&hv->hv_lock);
  eventfd = idr_remove(&hv->conn_to_evt, conn_id); // <= Memory block containing hv->hv_lock was released by other thread and reallocated by other thread.
  mutex_unlock(&hv->hv_lock); // <= Hence, __owner_task(owner) != current at this point.