linux-kernel - Re: [PATCH v2 11/43] KVM: Don't block+unblock when halt-polling is successful

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <880a5727-69d1-72a1-b129-b053781625ad@redhat.com>
Date:   Mon, 29 Nov 2021 20:18:50 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v2 11/43] KVM: Don't block+unblock when halt-polling is
 successful

On 11/29/21 19:55, Sean Christopherson wrote:
>> Still it does seem to be a race that happens when IS_RUNNING=true but
>> vcpu->mode == OUTSIDE_GUEST_MODE.  This patch makes the race easier to
>> trigger because it moves IS_RUNNING=false later.
> 
> Oh!  Any chance the bug only repros with preemption enabled?  That would explain
> why I don't see problems, I'm pretty sure I've only run AVIC with a PREEMPT=n.

Me too.

> svm_vcpu_{un}blocking() are called with preemption enabled, and avic_set_running()
> passes in vcpu->cpu.  If the vCPU is preempted and scheduled in on a different CPU,
> avic_vcpu_load() will overwrite the vCPU's entry with the wrong CPU info.

That would make a lot of sense.  avic_vcpu_load() can handle 
svm->avic_is_running = false, but avic_set_running still needs its body 
wrapped by preempt_disable/preempt_enable.

Fedora's kernel is CONFIG_PREEMPT_VOLUNTARY, but I know Maxim uses his 
own build so it would not surprise me if he used CONFIG_PREEMPT=y.

Paolo