Re: [PATCH] KVM: x86: Fully initialize 'struct kvm_lapic_irq' in kvm_pv_kick_cpu_op()

[Sean Christopherson]

On Tue, Jun 28, 2022, Vitaly Kuznetsov wrote:
> 'vector' and 'trig_mode' fields of 'struct kvm_lapic_irq' are left
> uninitialized in kvm_pv_kick_cpu_op(). While these fields are normally
> not needed for APIC_DM_REMRD, they're still referenced by
> __apic_accept_irq() for trace_kvm_apic_accept_irq(). Fully initialize
> the structure to avoid consuming random stack memory.
> 
> Fixes: a183b638b61c ("KVM: x86: make apic_accept_irq tracepoint more generic")
> Reported-by: syzbot+d6caa905917d353f0d07@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 567d13405445..8a98608dad4f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9340,15 +9340,17 @@ static int kvm_pv_clock_pairing(struct kvm_vcpu *vcpu, gpa_t paddr,
>   */
>  static void kvm_pv_kick_cpu_op(struct kvm *kvm, int apicid)
>  {
> -	struct kvm_lapic_irq lapic_irq;
> -
> -	lapic_irq.shorthand = APIC_DEST_NOSHORT;
> -	lapic_irq.dest_mode = APIC_DEST_PHYSICAL;
> -	lapic_irq.level = 0;
> -	lapic_irq.dest_id = apicid;
> -	lapic_irq.msi_redir_hint = false;
> +	struct kvm_lapic_irq lapic_irq = {
> +		.vector = 0,
> +		.delivery_mode = APIC_DM_REMRD,
> +		.dest_mode = APIC_DEST_PHYSICAL,
> +		.level = false,
> +		.trig_mode = 0,
> +		.shorthand = APIC_DEST_NOSHORT,
> +		.dest_id = apicid,
> +		.msi_redir_hint = false
> +	};

What if we rely on the compiler to zero-initialize omitted fields?  E.g.

	/*
	 * All other fields are unused for APIC_DM_REMRD, but may be consumed by
	 * common code, e.g. for tracing.  Defer initialization to the compiler.
	 */
	struct kvm_lapic_irq lapic_irq = {
		.delivery_mode = APIC_DM_REMRD,
		.dest_mode = APIC_DEST_PHYSICAL,
		.shorthand = APIC_DEST_NOSHORT,
		.dest_id = apicid,
	};

KVM doesn't actually care about the vector, level, trig_mode, etc... for its magic
magic DM_REMRD, i.e. using 0/false is completely arbitrary.  

>  
> -	lapic_irq.delivery_mode = APIC_DM_REMRD;
>  	kvm_irq_delivery_to_apic(kvm, NULL, &lapic_irq, NULL);
>  }
>  
> -- 
> 2.35.3
>