Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
Message-ID: <e0e127c8-1515-2ebf-f473-acc38d60a122@gmail.com>
Date:   Fri, 25 Mar 2022 21:11:03 +0300
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.6.1
Subject: Re: [RFC PATCH] KVM: x86/mmu: fix general protection fault in
 kvm_mmu_uninit_tdp_mmu
Content-Language: en-US
To:     Paolo Bonzini <pbonzini@redhat.com>, seanjc@google.com,
        vkuznets@redhat.com, wanpengli@tencent.com, jmattson@google.com
Cc:     x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzbot+717ed82268812a643b28@syzkaller.appspotmail.com
References: <20220325163815.3514-1-paskripkin@gmail.com>
 <53cc074f-350f-5fa8-1ee4-c33921f17cb1@redhat.com>
From:   Pavel Skripkin <paskripkin@gmail.com>
In-Reply-To: <53cc074f-350f-5fa8-1ee4-c33921f17cb1@redhat.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

Hi Paolo,

On 3/25/22 19:50, Paolo Bonzini wrote:
> On 3/25/22 17:38, Pavel Skripkin wrote:
>> Syzbot reported GPF in kvm_mmu_uninit_tdp_mmu(), which is caused by
>> passing NULL pointer to flush_workqueue().
>> 
>> tdp_mmu_zap_wq is allocated via alloc_workqueue() which may fail. There
>> is no error hanling and kvm_mmu_uninit_tdp_mmu() return value is simply
>> ignored. Even all kvm_*_init_vm() functions are void, so the easiest
>> solution is to check that tdp_mmu_zap_wq is valid pointer before passing
>> it somewhere.
> 
> Thanks for the analysis, but not scheduling the work item in
> tdp_mmu_schedule_zap_root is broken; you can't just let the roots
> survive (KVM uses its own workqueue because it needs to work item to
> complete has to flush it before kvm_mmu_zap_all_fast returns).
> 

Ah, I see, thanks for explanation.

I thought about propagating an error up to callers, but 
kvm_mmu_uninit_tdp_mmu() returns false with config disabled, so I 
decided to implement easiest fix w/o digging into details

sorry about that


With regards,
Pavel Skripkin