linux-kernel - Re: [PATCH v3 7/8] nSVM: use vmcb_ctrl_area_cached instead of vmcb_control_area in struct svm_nested

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Message-ID: <419f25f5-41ef-1389-1103-3bbe4aa4ddd3@redhat.com>
Date:   Fri, 22 Oct 2021 09:14:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org
Cc:     Maxim Levitsky <mlevitsk@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 7/8] nSVM: use vmcb_ctrl_area_cached instead of
 vmcb_control_area in struct svm_nested_state

On 11/10/21 16:37, Emanuele Giuseppe Esposito wrote:
> ZE))
>   		return -EFAULT;
> -	if (copy_to_user(&user_vmcb->control, &svm->nested.ctl,
> +	nested_copy_vmcb_cache_to_control(&ctl_temp, &svm->nested.ctl);
> +	if (copy_to_user(&user_vmcb->control, &ctl_temp,
>   			 sizeof(user_vmcb->control)))
>   		return -EFAULT;

This needs a memset of ctl_temp so that kernel memory contents are not
leaked to userspace.  However, it's also better to avoid large structs
on the stack, and do a quick kzalloc/kfree instead:

-	nested_copy_vmcb_cache_to_control(&ctl_temp, &svm->nested.ctl);
-	if (copy_to_user(&user_vmcb->control, &ctl_temp,
-			 sizeof(user_vmcb->control)))
+
+	ctl = kzalloc(sizeof(*ctl), GFP_KERNEL);
+	if (!ctl)
+		return -ENOMEM;
+	nested_copy_vmcb_cache_to_control(ctl, &svm->nested.ctl);
+	r = copy_to_user(&user_vmcb->control, ctl,
+			 sizeof(user_vmcb->control));
+	kfree(ctl);
+	if (r)
  		return -EFAULT;

I can do this change when committing too.

Paolo