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From: Maxim Levitsky <mlevitsk@redhat.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Nitesh Narayan Lal <nitesh@redhat.com>,
	linux-kernel@vger.kernel.org,
	Eduardo Habkost <ehabkost@redhat.com>
Subject: Re: [PATCH v2 4/4] KVM: x86: Fix stack-out-of-bounds memory access from ioapic_write_indirect()
Date: Wed, 25 Aug 2021 12:11:26 +0300	[thread overview]
Message-ID: <36b6656637d1e6aaa2ab5098f7ebc27644466294.camel@redhat.com> (raw)
In-Reply-To: <87h7fej5ov.fsf@vitty.brq.redhat.com>

On Wed, 2021-08-25 at 10:21 +0200, Vitaly Kuznetsov wrote:
> Maxim Levitsky <mlevitsk@redhat.com> writes:
> 
> > On Tue, 2021-08-24 at 16:42 +0200, Vitaly Kuznetsov wrote:
> ...
> > Not a classical review but,
> > I did some digital archaeology with this one, trying to understand what is going on:
> > 
> > 
> > I think that 16 bit vcpu bitmap is due to the fact that IOAPIC spec states that
> > it can address up to 16 cpus in physical destination mode.
> >  
> > In logical destination mode, assuming flat addressing and that logical id = 1 << physical id
> > which KVM hardcodes, it is also only possible to address 8 CPUs.
> >  
> > However(!) in flat cluster mode, the logical apic id is split in two.
> > We have 16 clusters and each have 4 CPUs, so it is possible to address 64 CPUs,
> > and unlike the logical ID, the KVM does honour cluster ID, 
> > thus one can stick say cluster ID 0 to any vCPU.
> >  
> >  
> > Let's look at ioapic_write_indirect.
> > It does:
> >  
> >     -> bitmap_zero(&vcpu_bitmap, 16);
> >     -> kvm_bitmap_or_dest_vcpus(ioapic->kvm, &irq, &vcpu_bitmap);
> >     -> kvm_make_scan_ioapic_request_mask(ioapic->kvm, &vcpu_bitmap); // use of the above bitmap
> >  
> >  
> > When we call kvm_bitmap_or_dest_vcpus, we can already overflow the bitmap,
> > since we pass all 8 bit of the destination even when it is physical.
> >  
> >  
> > Lets examine the kvm_bitmap_or_dest_vcpus:
> >  
> >   -> It calls the kvm_apic_map_get_dest_lapic which 
> >  
> >        -> for physical destinations, it just sets the bitmap, which can overflow
> >           if we pass it 8 bit destination (which basically includes reserved bits + 4 bit destination).
> >  
> >  
> >        -> For logical apic ID, it seems to truncate the result to 16 bit, which isn't correct as I explained
> >           above, but should not overflow the result.
> >  
> >   
> >    -> If call to kvm_apic_map_get_dest_lapic fails, it goes over all vcpus and tries to match the destination
> >        This can overflow as well.
> >  
> >  
> > I also don't like that ioapic_write_indirect calls the kvm_bitmap_or_dest_vcpus twice,
> > and second time with 'old_dest_id'
> >  
> > I am not 100%  sure why old_dest_id/old_dest_mode are needed as I don't see anything in the
> > function changing them.
> > I think only the guest can change them, so maybe the code deals with the guest changing them
> > while the code is running from a different vcpu?
> >  
> > The commit that introduced this code is 7ee30bc132c683d06a6d9e360e39e483e3990708
> > Nitesh Narayan Lal, maybe you remember something about it?
> >  
> 
> Before posting this patch I've contacted Nitesh privately, he's
> currently on vacation but will take a look when he gets back.
> 
> > Also I worry a lot about other callers of kvm_apic_map_get_dest_lapic
> >  
> > It is also called from kvm_irq_delivery_to_apic_fast, and from kvm_intr_is_single_vcpu_fast
> > and both seem to also use 'unsigned long' for bitmap, and then only use 16 bits of it.
> >  
> > I haven't dug into them, but these don't seem to be IOAPIC related and I think
> > can overwrite the stack as well.
> 
> I'm no expert in this code but when writing the patch I somehow
> convinced myself that a single unsigned long is always enough. I think
> that for cluster mode 'bitmap' needs 64-bits (and it is *not* a
> vcpu_bitmap, we need to convert). I may be completely wrong of course
> but in any case this is a different issue. In ioapic_write_indirect() we
> have 'vcpu_bitmap' which should certainly be longer than 64 bits.


This code which I mentioned in 'other callers' as far as I see is not IOAPIC related.
For regular local APIC all bets are off, any vCPU and apic ID are possible 
(xapic I think limits apic id to 255 but x2apic doesn't).

I strongly suspect that this code can overflow as well.

Best regards,
	Maxim Levitsky

> 



  reply	other threads:[~2021-08-25  9:11 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-23 14:30 [PATCH v2 0/4] KVM: Various fixes and improvements around kicking vCPUs Vitaly Kuznetsov
2021-08-23 14:30 ` [PATCH v2 1/4] KVM: Clean up benign vcpu->cpu data races when " Vitaly Kuznetsov
2021-08-23 14:30 ` [PATCH v2 2/4] KVM: Guard cpusmask NULL check with CONFIG_CPUMASK_OFFSTACK Vitaly Kuznetsov
2021-08-23 14:30 ` [PATCH v2 3/4] KVM: Optimize kvm_make_vcpus_request_mask() a bit Vitaly Kuznetsov
2021-08-23 14:30 ` [PATCH v2 4/4] KVM: x86: Fix stack-out-of-bounds memory access from ioapic_write_indirect() Vitaly Kuznetsov
2021-08-23 18:58   ` Eduardo Habkost
2021-08-24  7:13     ` Vitaly Kuznetsov
2021-08-24 14:23       ` Eduardo Habkost
2021-08-24 14:42         ` Vitaly Kuznetsov
2021-08-24 16:07           ` Maxim Levitsky
2021-08-24 17:40             ` Sean Christopherson
2021-08-25  8:26               ` Vitaly Kuznetsov
2021-08-25  8:21             ` Vitaly Kuznetsov
2021-08-25  9:11               ` Maxim Levitsky [this message]
2021-08-25  9:43                 ` Vitaly Kuznetsov
2021-08-25 10:41                   ` Maxim Levitsky
2021-08-25 13:19                   ` Eduardo Habkost
2021-08-26 12:40                     ` Vitaly Kuznetsov
2021-08-26 14:52                       ` Eduardo Habkost
2021-08-26 18:01                         ` Sean Christopherson
2021-08-26 18:13                           ` Eduardo Habkost
2021-08-26 19:27             ` Eduardo Habkost
2021-08-30 19:47             ` Nitesh Lal

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