linux-kernel - Re: [PATCH 3/5] KVM: selftests: Explicitly free CPUID array at end of Hyper-V CPUID test

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Message-ID: <Z4qVLU_hhwFHmic9@google.com>
Date: Fri, 17 Jan 2025 09:36:45 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Dongjie Zou <zoudongjie@...wei.com>, stable@...r.kernel
Subject: Re: [PATCH 3/5] KVM: selftests: Explicitly free CPUID array at end of
 Hyper-V CPUID test

On Fri, Jan 17, 2025, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> 
> > Explicitly free the array of CPUID entries at the end of the Hyper-V CPUID
> > test, mainly in anticipation of moving management of the array into the
> > main test helper.
> >
> > Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c b/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
> > index 9a0fcc713350..09f9874d7705 100644
> > --- a/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
> > +++ b/tools/testing/selftests/kvm/x86_64/hyperv_cpuid.c
> > @@ -164,6 +164,7 @@ int main(int argc, char *argv[])
> >  
> >  	hv_cpuid_entries = kvm_get_supported_hv_cpuid();
> >  	test_hv_cpuid(hv_cpuid_entries, kvm_cpu_has(X86_FEATURE_VMX));
> > +	free((void *)hv_cpuid_entries);
> 
> vcpu_get_supported_hv_cpuid() allocates memory for the resulting array
> each time, however, kvm_get_supported_hv_cpuid() was designed after
> what's now kvm_get_supported_cpuid() (afair) so it has an optimization
> to ask KVM just once:
> 
>         static struct kvm_cpuid2 *cpuid;
>         int kvm_fd;
> 
>         if (cpuid)
>                 return cpuid;
> 
>         cpuid = allocate_kvm_cpuid2(MAX_NR_CPUID_ENTRIES);
>         kvm_fd = open_kvm_dev_path_or_exit();
> 	...
> 
> and it seems that if we free hv_cpuid_entries here, next time we call
> kvm_get_supported_hv_cpuid() an already freed memory will be returned.
> This doesn't matter in in this patch as we're about to quit anyway but
> with the next one in the series it becomes problematic.

Ow.  I totally missed that.  I'll drop this patch, and then adjust the next one
to do:

	/*
	 * Note, the CPUID array returned by the system-scoped helper is a one-
	 * time allocation, i.e. must not be freed.
	 */
	if (vcpu)
		free((void *)hv_cpuid_entries);


I'll post a v2 once I've actually tested.

Thanks!