Re: [PATCH v2 1/2] KVM: MMIO: Lock coalesced device when checking for available entry

Avi Kivity
2011-07-18T13:12:02Z

On 07/18/2011 03:58 PM, Sasha Levin wrote:
> On Mon, 2011-07-18 at 15:29 +0300, Avi Kivity wrote:
> > On 07/18/2011 03:03 PM, Sasha Levin wrote:
> > > On Mon, 2011-07-18 at 14:43 +0300, Avi Kivity wrote:
> > > > On 07/18/2011 01:15 PM, Sasha Levin wrote:
> > > > > On Mon, 2011-07-18 at 12:50 +0300, Avi Kivity wrote:
> > > > > > On 07/18/2011 12:29 PM, Sasha Levin wrote:
> > > > > > > > Hmm. This means we take the lock for every I/O, whether it hits
> > > > > > > > coalesced mmio or not.
> > > > > > > >
> > > > > > > > We need to do the range check before taking the lock and the space check
> > > > > > > > after taking the lock.
> > > > > > > >
> > > > > > >
> > > > > > > I'll fix that.
> > > > > > >
> > > > > > > Shouldn't the range check be also locked somehow? Currently it is
> > > > > > > possible that a coalesced region was removed while we are checking the
> > > > > > > ranges, and we won't issue a mmio exit as the host expects
> > > > > >
> > > > > > It's "locked" using rcu.
> > > > > >
> > > > >
> > > > > Where is that happening?
> > > > >
> > > > > All the coalesced zones are stored under the coalesced "device" in a
> > > > > simple array. When adding and removing zones, kvm->slots_lock is taken -
> > > > > I don't see anything which prevents a range check during zone removal
> > > > > unless slots_lock prevents IO.
> > > >
> > > > Range check during slot removal is legal. While you are removing a
> > > > slot, a concurrent write may hit or miss the slot; it doesn't matter.
> > > >
> > > > Userspace should flush the coalesced mmio buffer after removal to ensure
> > > > there are no pending writes.
> > > >
> > >
> > > But the write may hit a non-existent slot.
> > >
> > > Something like this:
> > >
> > > Thread 1 Thread 2
> > > ----------------------------------
> > > Check range |
> > > Found slot |
> > > | Remove slot
> > > | Flush buffer
> > > Get spinlock |
> > > Write to buffer |
> > >
> >
> > Cannot happen, due to rcu. The "remove slot" step waits until all rcu
> > readers are gone.
> >
> > In other words: it's magic.
> >
>
> I might be missing something, but I don't see anything rcu related in
> anything within /virt/kvm/coalesced_mmio.c or in
> kvm_vm_ioctl_unregister_coalesced_mmio() specifically.
>
> Where is rcu invoked on the zones array?
>
> All I see is a simple array and counter declared as such:
>
> int nb_zones;
> struct kvm_coalesced_mmio_zone zone[KVM_COALESCED_MMIO_ZONE_MAX];
>
> And in the register/unregister functions it's a simple array manipulation.

Er, kvm_io_bus_register() does the rcu stuff. But
kvm_register_coalesced_mmio() doesn't call it! Instead it's just one
device on the bus that decodes all those offsets.

In other words: it's broken.

Luckily, it's not exploitable, since the memory is static wrt the
lifetime of the guest.

We should probably make it separate kvm_io_devices so we can reuse the
existing locking.

--
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