Re: [PATCH v2 1/2] KVM: cpuid: Fix read/write out-of-bounds vulnerability in cpuid emulation

[Paolo Bonzini]

On 08/06/2017 10:22, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> 
> If "i" is the last element in the vcpu->arch.cpuid_entries[] array, it 
> potentially can be exploited the vulnerability. this will out-of-bounds 
> read and write the unused memory in host OS.
> 
> As Paolo pointed:
> 
>>>  	/* when no next entry is found, the current entry[i] is reselected */
>>> -	for (j = i + 1; ; j = (j + 1) % nent) {
>>> -		struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
>>> -		if (ej->function == e->function) {
>>
>> It reads ej->maxphyaddr, which is user controlled.
>>
>>> -			ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
>>
>> After cpuid_entries there is
>>
>>        int maxphyaddr;
>>        struct x86_emulate_ctxt emulate_ctxt;  /* 16-byte aligned */
>>
>> So indeed we have:
>>
>> - cpuid_entries at offset 1B50 (6992)
>> - maxphyaddr at offset 27D0 (6992 + 3200 = 10192)
>> - padding at 27D4...27DF
>> - emulate_ctxt at 27E0
>>
>> So this indeed writes in the padding.  Pfew, writing the ops field of
>> emulate_ctxt would have been much worse.
> 
> This patch fixes it by modding the index to avoid the out-of-bounds. At 
> the worst case, i == j and ej->function == e->function, the loop can bail
> out.
> 
> Reported-by: Moguofang <moguofang@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Moguofang <moguofang@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> v1 -> v2:
>  * update patch description

Queued, thanks.

Paolo

> 
>  arch/x86/kvm/cpuid.c | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index a181ae7..b927a42 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -779,19 +779,20 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
>  
>  static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i)
>  {
> +	int j = i, nent = vcpu->arch.cpuid_nent;
>  	struct kvm_cpuid_entry2 *e = &vcpu->arch.cpuid_entries[i];
> -	int j, nent = vcpu->arch.cpuid_nent;
> +	struct kvm_cpuid_entry2 *ej;
>  
>  	e->flags &= ~KVM_CPUID_FLAG_STATE_READ_NEXT;
>  	/* when no next entry is found, the current entry[i] is reselected */
> -	for (j = i + 1; ; j = (j + 1) % nent) {
> -		struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
> -		if (ej->function == e->function) {
> -			ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
> -			return j;
> -		}
> -	}
> -	return 0; /* silence gcc, even though control never reaches here */
> +	do {
> +		j = (j + 1) % nent;
> +		ej = &vcpu->arch.cpuid_entries[j];
> +	} while(ej->function != e->function);
> +
> +	ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
> +
> +	return j;
>  }
>  
>  /* find an entry with matching function, matching index (if needed), and that
>