Re: [PATCH 09/10] KVM: SVM: Drop redundant check in AVIC code on ID during vCPU creation

[Maxim Levitsky]

У вт, 2023-08-15 у 14:35 -0700, Sean Christopherson пише:
> Drop avic_get_physical_id_entry()'s compatibility check on the incoming
> ID, as its sole caller, avic_init_backing_page(), performs the exact same
> check.  Drop avic_get_physical_id_entry() entirely as the only remaining
> functionality is getting the address of the Physical ID table, and
> accessing the array without an immediate bounds check is kludgy.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/avic.c | 28 ++++++----------------------
>  1 file changed, 6 insertions(+), 22 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 3b2d00d9ca9b..6803e2d7bc22 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -263,26 +263,12 @@ void avic_init_vmcb(struct vcpu_svm *svm, struct vmcb *vmcb)
>  		avic_deactivate_vmcb(svm);
>  }
>  
> -static u64 *avic_get_physical_id_entry(struct kvm_vcpu *vcpu,
> -				       unsigned int index)
> -{
> -	u64 *avic_physical_id_table;
> -	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
> -
> -	if ((!x2avic_enabled && index > AVIC_MAX_PHYSICAL_ID) ||
> -	    (index > X2AVIC_MAX_PHYSICAL_ID))
> -		return NULL;

While removing this code doesn't introduce a bug, it does make it less safe,
because new code just blindly trusts that vcpu_id will never be out of bounds
of the physical id table.

Bugs happen and that can and will someday happen.

> -
> -	avic_physical_id_table = page_address(kvm_svm->avic_physical_id_table_page);
> -
> -	return &avic_physical_id_table[index];
> -}
> -
>  static int avic_init_backing_page(struct kvm_vcpu *vcpu)
>  {
> -	u64 *entry, new_entry;
> +	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +	u64 *table, new_entry;
>  	int id = vcpu->vcpu_id;
> -	struct vcpu_svm *svm = to_svm(vcpu);
>  
>  	/*
>  	 * Inhibit AVIC if the vCPU ID is bigger than what is supported by AVIC
> @@ -318,15 +304,13 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
>  	}
>  
>  	/* Setting AVIC backing page address in the phy APIC ID table */
> -	entry = avic_get_physical_id_entry(vcpu, id);
> -	if (!entry)
> -		return -EINVAL;
> +	table = page_address(kvm_svm->avic_physical_id_table_page);
>  
>  	new_entry = avic_get_backing_page_address(svm) |
>  		    AVIC_PHYSICAL_ID_ENTRY_VALID_MASK;
> -	WRITE_ONCE(*entry, new_entry);

Here I prefer to at least have an assert that id is in bounds of a page 
(at least less than 512) so that a bug will not turn into a security
issue by overflowing the buffer.

> +	WRITE_ONCE(table[id], new_entry);
>  
> -	svm->avic_physical_id_cache = entry;
> +	svm->avic_physical_id_cache = &table[id];
>  
>  	return 0;
>  }

Best regards,
	Maxim Levitsky