Re: [PATCH v2] x86/perf: Use RET0 as default for guest_get_msrs to handle "no PMU" case

[Jim Mattson]

On Tue, Mar 9, 2021 at 9:10 AM Sean Christopherson <seanjc@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Reported-by: syzbot+cce9ef2dd25246f815ee@xxxxxxxxxxxxxxxxxxxxxxxxx
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>
> 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.]