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Date:   Thu, 30 Aug 2018 13:01:21 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Liran Alon <liran.alon@...cle.com>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH v2] KVM: LAPIC: Fix pv ipis out-of-bounds access

2018-08-30 10:03+0800, Wanpeng Li:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> 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, 
> ignoring destinations that are not present and delivering the rest. We also check 
> whether or not map->phys_map[min + i] is NULL since the max_apic_id is set to the 
> max apic id, some phys_map maybe NULL when apic id is sparse, especially kvm 
> unconditionally set max_apic_id to 255 to reserve enough space for any xAPIC ID.
> 
> Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
> Reviewed-by: Liran Alon <liran.alon@...cle.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Liran Alon <liran.alon@...cle.com>
> Cc: Dan Carpenter <dan.carpenter@...cle.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> v1 -> v2:
>  * add min > map->max_apic_id check
>  * change min to u32
>  * add min((u32)BITS_PER_LONG, (map->max_apic_id - min + 1))
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> @@ -548,7 +548,7 @@ int kvm_apic_set_irq(struct kvm_vcpu *vcpu, struct kvm_lapic_irq *irq,
>  }
>  
>  int kvm_pv_send_ipi(struct kvm *kvm, unsigned long ipi_bitmap_low,
> -    		    unsigned long ipi_bitmap_high, int min,
> +		    unsigned long ipi_bitmap_high, u32 min,
>  		    unsigned long icr, int op_64_bit)
>  {
>  	int i;
> @@ -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 (min > map->max_apic_id)
> +		goto out;
>  	/* 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);
> +	for_each_set_bit(i, &ipi_bitmap_low,
> +		min((u32)BITS_PER_LONG, (map->max_apic_id - min + 1))) {
> +		if (map->phys_map[min + i]) {
> +			vcpu = map->phys_map[min + i]->vcpu;
> +			count += kvm_apic_set_irq(vcpu, &irq, NULL);
> +		}
>  	}
>  
>  	min += cluster_size;

We need a second

  if (min > map->max_apic_id)
  	goto out;

here.  I will add it while applying if there are no other change
requests.

> -	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);
> +	for_each_set_bit(i, &ipi_bitmap_high,
> +		min((u32)BITS_PER_LONG, (map->max_apic_id - min + 1))) {
> +		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
> 

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