linux-kernel - Re: [PATCH] KVM: vPMU: Use atomic bit operations for global

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] [day] [month] [year] [list]

Date:   Mon, 11 Sep 2023 16:42:46 -0700
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Dapeng Mi <dapeng1.mi@...el.com>,
        Jim Mattson <jmattson@...gle.com>, Like Xu <likexu@...cent.com>
Subject: Re: [PATCH] KVM: vPMU: Use atomic bit operations for global_status

On Mon, Sep 11, 2023 at 11:09 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Mon, Sep 11, 2023, Mingwei Zhang wrote:
> > On Mon, Sep 11, 2023 at 8:01 AM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > On Mon, Sep 11, 2023, Mingwei Zhang wrote:
> > > > Use atomic bit operations for pmu->global_status because it may suffer from
> > > > race conditions between emulated overflow in KVM vPMU and PEBS overflow in
> > > > host PMI handler.
> > >
> > > Only if the host PMI occurs on a different pCPU, and if that can happen don't we
> > > have a much larger problem?
> >
> > Why on different pCPU?  For vPMU, I think there is always contention
> > between the vCPU thread and the host PMI handler running on the same
> > pCPU, no?
>
> A non-atomic instruction can't be interrupted by an NMI, or any other event, so
> I don't see how switching to atomic operations fixes anything unless the NMI comes
> in on a different pCPU.

You are right. I realize that. The race condition has to happen
concurrently from two different pCPUs. This happens to
pmu->reprogram_nmi but not pmu->global_status. The critical stuff we
care about should be re-entrancy issues for __kvm_perf_overflow() and
some state maintenance issues like avoiding duplicate NMI injection.

That concludes that we don't need this change.

Thanks
-Mingwei