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From:   Jim Mattson <jmattson@google.com>
Date:   Tue, 9 Mar 2021 10:26:38 -0800
Message-ID: <CALMp9eRmxLTXdVoweUpZPaSyY7O4HK=KcLT243TbV4MpE8Dttg@mail.gmail.com>
Subject: Re: [PATCH v2] x86/perf: Use RET0 as default for guest_get_msrs to
 handle "no PMU" case
To:     Sean Christopherson <seanjc@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        Like Xu <like.xu@linux.intel.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        syzbot+cce9ef2dd25246f815ee@syzkaller.appspotmail.com
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On Tue, Mar 9, 2021 at 9:10 AM Sean Christopherson <seanjc@google.com> wrote:
>
> Initialize x86_pmu.guest_get_msrs to return 0/NULL to handle the "nop"
> case.  Patching in perf_guest_get_msrs_nop() during setup does not work
> if there is no PMU, as setup bails before updating the static calls,
> leaving x86_pmu.guest_get_msrs NULL and thus a complete nop.  Ultimately,
> this causes VMX abort on VM-Exit due to KVM putting random garbage from
> the stack into the MSR load list.
>
> Add a comment in KVM to note that nr_msrs is valid if and only if the
> return value is non-NULL.
>
> Fixes: abd562df94d1 ("x86/perf: Use static_call for x86_pmu.guest_get_msrs")
> Cc: Like Xu <like.xu@linux.intel.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Jim Mattson <jmattson@google.com>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Reported-by: syzbot+cce9ef2dd25246f815ee@syzkaller.appspotmail.com
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>
> v2:
>  - Use __static_call_return0 to return NULL instead of manually checking
>    the hook at invocation.  [Peter]
>  - Rebase to tip/sched/core, commit 4117cebf1a9f ("psi: Optimize task
>    switch inside shared cgroups").
>
...
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 50810d471462..32cf8287d4a7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6580,8 +6580,8 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
>         int i, nr_msrs;
>         struct perf_guest_switch_msr *msrs;
>
> +       /* Note, nr_msrs may be garbage if perf_guest_get_msrs() returns NULL. */

You could drop the scary comment with a profligate initialization of
nr_msrs to 0.

[Apologies to those seeing this twice. I blame gmail.]