Re: [RFC PATCH] kvm: x86/vmx: Use kzalloc for cached_vmcs12

[Sean Christopherson]

On Mon, Jan 14, 2019 at 03:47:28PM -0800, 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.

Is the leak specific to vmx_set_nested_state(), e.g. can we zero out
the memory if copy_from_user() fails instead of taking the hit on every
allocation?

> The email from Syszkaller led to a discussion about a patch in early
> November on the KVM list (I've made this a reply to that thread), but
> the current upstream kernel still has kmalloc instead of kzalloc for
> cached_vmcs12 and cached_shadow_vmcs12. This RFC proposes changing to
> kzalloc for defense in depth.
> 
> Tested: rebuilt but not tested, since this is an RFC
> 
> Reported-by: syzbot+ded1696f6b50b615b630@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Tom Roeder <tmroeder@xxxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/nested.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 2616bd2c7f2c7..ad46667042c7a 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;

Obviously not your code, but why do we allocate VMCS12_SIZE instead of
sizeof(struct vmcs12)?  I get why we require userspace to reserve the
full 4k, but I don't understand why KVM needs to allocate the reserved
bytes internally.

> -	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;
>  
> -- 
> 2.20.1.97.g81188d93c3-goog
>