Re: [PATCH 1/4] KVM: SVM: nested: Don't allocate VMCB structures on stack

From: Joerg Roedel <joro@8bytes.org>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Joerg Roedel <jroedel@suse.de>
Subject: Re: [PATCH 1/4] KVM: SVM: nested: Don't allocate VMCB structures on stack
Date: Thu, 30 Jul 2020 14:38:52 +0200	[thread overview]
Message-ID: <20200730123852.GB3257@8bytes.org> (raw)
In-Reply-To: <20200729151454.GB27751@linux.intel.com>

Hi Sean,

thanks for your review!

On Wed, Jul 29, 2020 at 08:14:55AM -0700, Sean Christopherson wrote:
> On Wed, Jul 29, 2020 at 03:22:31PM +0200, Joerg Roedel wrote:
> Speaking of too large, would it be overly paranoid to add:
> 
>   BUILD_BUG_ON(sizeof(struct vmcb_control_area) + sizeof(struct vmcb_save_area) <
> 	       KVM_STATE_NESTED_SVM_VMCB_SIZE)
> 
> More so for documentation than for any real concern that the SVM architecture
> will do something silly, e.g. to make it obvious that patch 2 in this series
> won't break backwards compatibility.

The check should actually be '>', but then it makes sense. The control-
and save-area together are still way smaller than 4k. I will add the
check for '>' to this patch.

> > +	ret = -EFAULT;
> > +	if (copy_from_user(ctl, &user_vmcb->control, sizeof(ctl)))
> 
> The sizeof() calc is wrong, this is now calculating the size of the pointer,
> not the size of the struct.  It'd need to be sizeof(*ctl).
> 
> > +		goto out_free;
> > +	if (copy_from_user(save, &user_vmcb->save, sizeof(save)))
> 
> Same bug here.

Thanks, fixed that.

Regards,

	Joerg