Re: [PATCH] KVM: LAPIC: Fix pv ipis out-of-bounds access

[Liran Alon]

> On 29 Aug 2018, at 8:52, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> 
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> 
> Dan Carpenter reported that the untrusted data returns from kvm_register_read()
> results in the following static checker warning:
>  arch/x86/kvm/lapic.c:576 kvm_pv_send_ipi()
>  error: buffer underflow 'map->phys_map' 's32min-s32max'
> 
> KVM guest can easily trigger this by executing the following assembly sequence 
> in Ring0:
> 
> mov $10, %rax
> mov $0xFFFFFFFF, %rbx
> mov $0xFFFFFFFF, %rdx
> mov $0, %rsi
> vmcall
> 
> As this will cause KVM to execute the following code-path:
> vmx_handle_exit() -> handle_vmcall() -> kvm_emulate_hypercall() -> kvm_pv_send_ipi()
> which will reach out-of-bounds access.
> 
> This patch fixes it by adding a check to kvm_pv_send_ipi() against map->max_apic_id 
> and also checking whether or not map->phys_map[min + i] is NULL since the max_apic_id 
> is set according to the max apic id, however, some phys_map maybe NULL when apic id 
> is sparse, in addition, kvm also unconditionally set max_apic_id to 255 to reserve 
> enough space for any xAPIC ID.
> 
> Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Liran Alon <liran.alon@xxxxxxxxxx>
> Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
> arch/x86/kvm/lapic.c | 17 +++++++++++++----
> 1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 0cefba2..86e933c 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -571,18 +571,27 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned long ipi_bitmap_low,
> 	rcu_read_lock();
> 	map = rcu_dereference(kvm->arch.apic_map);
> 
> +	if (unlikely((s32)(map->max_apic_id - __fls(ipi_bitmap_low)) < min))
> +		goto out;

I personally think âif ((min + __fls(ipi_bitmap_low)) > map->max_apic_id)â is more readable.
But thatâs just a matter of taste :)

> 	/* Bits above cluster_size are masked in the caller.  */
> 	for_each_set_bit(i, &ipi_bitmap_low, BITS_PER_LONG) {
> -		vcpu = map->phys_map[min + i]->vcpu;
> -		count += kvm_apic_set_irq(vcpu, &irq, NULL);
> +		if (map->phys_map[min + i]) {
> +			vcpu = map->phys_map[min + i]->vcpu;
> +			count += kvm_apic_set_irq(vcpu, &irq, NULL);
> +		}
> 	}
> 
> 	min += cluster_size;
> +	if (unlikely((s32)(map->max_apic_id - __fls(ipi_bitmap_high)) < min))
> +		goto out;
> 	for_each_set_bit(i, &ipi_bitmap_high, BITS_PER_LONG) {
> -		vcpu = map->phys_map[min + i]->vcpu;
> -		count += kvm_apic_set_irq(vcpu, &irq, NULL);
> +		if (map->phys_map[min + i]) {
> +			vcpu = map->phys_map[min + i]->vcpu;
> +			count += kvm_apic_set_irq(vcpu, &irq, NULL);
> +		}
> 	}
> 
> +out:
> 	rcu_read_unlock();
> 	return count;
> }
> -- 
> 2.7.4
> 

Reviewed-By: Liran Alon <liran.alon@xxxxxxxxxx>