Re: [PATCH v4 28/32] KVM: SVM: Require logical ID to be power-of-2 for AVIC entry

From: Maxim Levitsky <mlevitsk@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Alejandro Jimenez <alejandro.j.jimenez@oracle.com>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
	Li RongQing <lirongqing@baidu.com>
Subject: Re: [PATCH v4 28/32] KVM: SVM: Require logical ID to be power-of-2 for AVIC entry
Date: Wed, 04 Jan 2023 20:34:28 +0200	[thread overview]
Message-ID: <ac7beb326b44a45d5561803e62b8b94cd758a45d.camel@redhat.com> (raw)
In-Reply-To: <Y7W/NinWOnJMtUk8@google.com>

On Wed, 2023-01-04 at 18:02 +0000, Sean Christopherson wrote:
> On Thu, Dec 29, 2022, mlevitsk@redhat.com wrote:
> > On Fri, 2022-12-09 at 00:00 +0200, Maxim Levitsky wrote:
> > > On Sat, 2022-10-01 at 00:59 +0000, Sean Christopherson wrote:
> > > > +	} else {
> > > > +		cluster = (ldr >> 4) << 2;
> > > > +		if (cluster >= 0xf)
> > > >  			return NULL;
> > > > -	} else { /* cluster */
> > > > -		int cluster = (dlid & 0xf0) >> 4;
> > > > -		int apic = ffs(dlid & 0x0f) - 1;
> > > > -
> > > > -		if ((apic < 0) || (apic > 7) ||
> > > > -		    (cluster >= 0xf))
> > > > -			return NULL;
> > > > -		index = (cluster << 2) + apic;
> > > > +		ldr &= 0xf;
> > > >  	}
> > > > +	if (!ldr || !is_power_of_2(ldr))
> > > > +		return NULL;
> > > > +
> > > > +	index = __ffs(ldr);
> > > > +	if (WARN_ON_ONCE(index > 7))
> > > > +		return NULL;
> > > > +	index += (cluster << 2);
> > > >  
> > > >  	logical_apic_id_table = (u32 *) page_address(kvm_svm->avic_logical_id_table_page);
> > > >  
> > > 
> > > Looks good.
> > 
> > I hate to say it but this patch has a bug:
> > 
> > We have both 'cluster = (ldr >> 4) << 2' and then 'index += (cluster << 2)'
> > 
> > One of the shifts has to go.
> 
> The first shift is wrong.  The "cluster >= 0xf" check needs to be done on the actual
> cluster.  The "<< 2", a.k.a. "* 4", is specific to indexing the AVIC table.
> 
> Thanks!
> 

Yep, agree.

(I mean technically you can remove the second shift and do the check before doing the first shift, that
is why I told you that one of the shifts has to go)

Best regards,
	Maxim Levitsky