linux-kernel - Re: [RFC Part2 PATCH v3 19/26] KVM: svm: Add support for SEV GUEST

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Date:   Thu, 14 Sep 2017 12:35:39 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        \"Radim Krčmář\" <rkrcmar@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [RFC Part2 PATCH v3 19/26] KVM: svm: Add support for SEV
 GUEST_STATUS command

On Mon, Jul 24, 2017 at 03:02:56PM -0500, Brijesh Singh wrote:
> The command is used for querying the SEV guest status.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> ---
>  arch/x86/kvm/svm.c | 38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 7a77197..21f85e1 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -6024,6 +6024,40 @@ static int sev_launch_finish(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  	return ret;
>  }
>  
> +static int sev_guest_status(struct kvm *kvm, struct kvm_sev_cmd *argp)
> +{
> +	struct kvm_sev_guest_status params;
> +	struct sev_data_guest_status *data;
> +	int ret;
> +
> +	if (!sev_guest(kvm))
> +		return -ENOTTY;
> +
> +	if (copy_from_user(&params, (void *) argp->data,
> +				sizeof(struct kvm_sev_guest_status)))

Let me try to understand what's going on here. You copy user data into
params...

> +		return -EFAULT;
> +
> +	data = kzalloc(sizeof(*data), GFP_KERNEL);
> +	if (!data)
> +		return -ENOMEM;
> +
> +	data->handle = sev_get_handle(kvm);
> +	ret = sev_issue_cmd(kvm, SEV_CMD_GUEST_STATUS, data, &argp->error);
> +	if (ret)
> +		goto e_free;
> +
> +	params.policy = data->policy;
> +	params.state = data->state;
> +	params.handle = data->handle;

... *overwrite* the copied data which means, the copy meant *absolutely*
*nothing* at all! ...

Also, why does userspace need to know the firmware ->handle?

> +
> +	if (copy_to_user((void *) argp->data, &params,
> +				sizeof(struct kvm_sev_guest_status)))

... and here you copy it back. And the caller svm_memory_encryption_op()
copies sev_cmd yet again! Probably for the sev_cmd.error value.
Ok, looking at other commands, they use more members like fd in
sev_guest_init(), for example.

But please audit all that shuffling of data back and forth and bring it
down to a mininum. No useless work pls.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--