linux-kernel - Re: [PATCH v2] kvm: x86/vmx: Use kzalloc for cached

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Date:   Fri, 25 Jan 2019 18:52:02 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tom Roeder <tmroeder@...gle.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Liran Alon <liran.alon@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+ded1696f6b50b615b630@...kaller.appspotmail.com
Subject: Re: [PATCH v2] kvm: x86/vmx: Use kzalloc for cached_vmcs12

On 24/01/19 22:48, Tom Roeder wrote:
> This changes the allocation of cached_vmcs12 to use kzalloc instead of
> kmalloc. This removes the information leak found by Syzkaller (see
> Reported-by) in this case and prevents similar leaks from happening
> based on cached_vmcs12.
> 
> It also changes vmx_get_nested_state to copy out the full 4k VMCS12_SIZE
> in copy_to_user rather than only the size of the struct.
> 
> Tested: rebuilt against head, booted, and ran the syszkaller repro
>   https://syzkaller.appspot.com/text?tag=ReproC&x=174efca3400000 without
>   observing any problems.
> 
> Reported-by: syzbot+ded1696f6b50b615b630@...kaller.appspotmail.com
> Signed-off-by: Tom Roeder <tmroeder@...gle.com>
> ---
> Changelog since v1:
> - Changed the copy_to_user calls in vmx_get_nested_state to copy the
>   full 4k buffer.
> 
>  arch/x86/kvm/vmx/nested.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 2616bd2c7f2c7..ce81539238547 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4140,11 +4140,11 @@ static int enter_vmx_operation(struct kvm_vcpu *vcpu)
>  	if (r < 0)
>  		goto out_vmcs02;
>  
> -	vmx->nested.cached_vmcs12 = kmalloc(VMCS12_SIZE, GFP_KERNEL);
> +	vmx->nested.cached_vmcs12 = kzalloc(VMCS12_SIZE, GFP_KERNEL);
>  	if (!vmx->nested.cached_vmcs12)
>  		goto out_cached_vmcs12;
>  
> -	vmx->nested.cached_shadow_vmcs12 = kmalloc(VMCS12_SIZE, GFP_KERNEL);
> +	vmx->nested.cached_shadow_vmcs12 = kzalloc(VMCS12_SIZE, GFP_KERNEL);
>  	if (!vmx->nested.cached_shadow_vmcs12)
>  		goto out_cached_shadow_vmcs12;
>  
> @@ -5263,13 +5263,17 @@ static int vmx_get_nested_state(struct kvm_vcpu *vcpu,
>  			copy_shadow_to_vmcs12(vmx);
>  	}
>  
> -	if (copy_to_user(user_kvm_nested_state->data, vmcs12, sizeof(*vmcs12)))
> +	/*
> +	 * Copy over the full allocated size of vmcs12 rather than just the size
> +	 * of the struct.
> +	 */
> +	if (copy_to_user(user_kvm_nested_state->data, vmcs12, VMCS12_SIZE))
>  		return -EFAULT;
>  
>  	if (nested_cpu_has_shadow_vmcs(vmcs12) &&
>  	    vmcs12->vmcs_link_pointer != -1ull) {
>  		if (copy_to_user(user_kvm_nested_state->data + VMCS12_SIZE,
> -				 get_shadow_vmcs12(vcpu), sizeof(*vmcs12)))
> +				 get_shadow_vmcs12(vcpu), VMCS12_SIZE))
>  			return -EFAULT;
>  	}
>  
> 

Queued, thanks.

Paolo