linux-kernel - Re: [PATCH] KVM: cpuid: Fix read/write out-of-bounds vulnerability in cpuid emulation

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Date:   Wed, 7 Jun 2017 14:55:05 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Moguofang <moguofang@...wei.com>
Subject: Re: [PATCH] KVM: cpuid: Fix read/write out-of-bounds vulnerability in
 cpuid emulation



On 07/06/2017 14:30, Wanpeng Li wrote:
> 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) {

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.

Thanks,

Paolo

> -			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
>