linux-kernel - Re: [RFC PATCH] KVM: x86/mmu: fix general protection fault in kvm_mmu_uninit_tdp

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Message-ID: <e0e127c8-1515-2ebf-f473-acc38d60a122@gmail.com>
Date:   Fri, 25 Mar 2022 21:11:03 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, seanjc@...gle.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+717ed82268812a643b28@...kaller.appspotmail.com
Subject: Re: [RFC PATCH] KVM: x86/mmu: fix general protection fault in
 kvm_mmu_uninit_tdp_mmu

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