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Subject: Re: [PATCH 2/2] KVM: Fix rcu splat if vm creation fails
To:     Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
References: <1572848879-21011-1-git-send-email-wanpengli@tencent.com>
 <1572848879-21011-2-git-send-email-wanpengli@tencent.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Date:   Mon, 4 Nov 2019 12:18:10 +0100
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On 04/11/19 07:27, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> Reported by syzkaller:
> 
>    =============================
>    WARNING: suspicious RCU usage
>    -----------------------------
>    ./include/linux/kvm_host.h:536 suspicious rcu_dereference_check() usage!
>    
>    other info that might help us debug this:
> 
>    rcu_scheduler_active = 2, debug_locks = 1
>    no locks held by repro_11/12688.
>     
>    stack backtrace:
>    Call Trace:
>     dump_stack+0x7d/0xc5
>     lockdep_rcu_suspicious+0x123/0x170
>     kvm_dev_ioctl+0x9a9/0x1260 [kvm]
>     do_vfs_ioctl+0x1a1/0xfb0
>     ksys_ioctl+0x6d/0x80
>     __x64_sys_ioctl+0x73/0xb0
>     do_syscall_64+0x108/0xaa0
>     entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> Commit a97b0e773e4 (kvm: call kvm_arch_destroy_vm if vm creation fails)
> sets users_count to 1 before kvm_arch_init_vm(), however, if kvm_arch_init_vm()
> fails, we need to dec this count. Or, we can move the sets refcount after 
> kvm_arch_init_vm().

I don't understand this one, hasn't

        WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));

decreased the conut already?  With your patch the refcount would then
underflow.

Paolo