linux-kernel - Re: [PATCH] KVM: x86: ioapic: Fix level-triggered EOI and userspace IOAPIC reconfigure race

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Date:   Wed, 07 Dec 2022 09:42:15 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Adamos Ttofari <attofari@...zon.de>, Thomas Huth <thuth@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Nikita Leshenko <nikita.leshchenko@...cle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: ioapic: Fix level-triggered EOI and userspace
 IOAPIC reconfigure race

On Wed, 2022-12-07 at 09:13 +0000, Adamos Ttofari wrote:
> When split-irqchip is used KVM uses ioapic_handled_vectors to identify
> which vectors require an exit to userspace IOAPIC. Unfortunately, when the
> IOAPIC is reconfigured while the interrupt is being handled, it will use
> the newest configuration; therefore, the EOI will not be delivered to
> IOAPIC.
> 
> A previous commit 0fc5a36dd6b3
> ("KVM: x86: ioapic: Fix level-triggered EOI and IOAPIC reconfigure race")
> fixed the race for kernel ioapic, but the issue still persists for
> userspace IOAPIC:
> 
> 1) Userspace IOAPIC sends a level triggered interrupt to VCPU0.
> 2) VCPU0's handler reconfigures the IOAPIC to route the interrupts to
>    VCPU1. (This can cause userspace IOAPIC to commit a new routing table,
>    eventually leading KVM to unset the vector in ioapic_handled_vectors)
> 3) VCPU0 triggers an EOI, and it's not delivered to userspace IOAPIC
>    because the vector bit is not set in ioapic_handled_vectors.
> 4) The loss of EOI, leaves remote_irr in IOAPIC set. Eventually blocking
>    new interrupts.
> 
> To avoid the above scenario, we should apply a similar fix like
> commit 0fc5a36dd6b3 ("KVM: x86: ioapic: Fix level-triggered EOI and IOAPIC
> reconfigure race") Which is to add all pending and running vectors to
> ioapic_handled_vectors.
> 
> Fixes: 0fc5a36dd6b3 ("KVM: x86: ioapic: Fix level-triggered EOI and IOAPIC reconfigure race")
> 
> Signed-off-by: Adamos Ttofari <attofari@...zon.de>

Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>

Thanks.

This should fix https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/issues/1
shouldn't it?

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