Re: [RFC PATCH 5/6] kvm: Re-introduce memslots->nmemslots

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Dec 05, 2012 at 04:02:53PM -0700, Alex Williamson wrote:
> On Wed, 2012-12-05 at 19:26 -0200, Marcelo Tosatti wrote:
> > On Mon, Dec 03, 2012 at 04:39:36PM -0700, Alex Williamson wrote:
> > > struct kvm_memory_slot is currently 52 bytes (LP64), not counting the
> > > arch data.  On x86 this means the memslot array to support a tiny 32+3
> > > entries (user+private) is over 2k.  We'd like to support more slots
> > > so that we can support more assigned devices, but it doesn't make
> > > sense to penalize everyone by using a statically allocated array.
> > > This allows us to start introducing a grow-able array.
> > > 
> > > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > > ---
> > >  arch/ia64/kvm/kvm-ia64.c     |    2 +-
> > >  arch/powerpc/kvm/book3s_hv.c |    2 +-
> > >  arch/x86/kvm/vmx.c           |    1 +
> > >  arch/x86/kvm/x86.c           |    4 +++-
> > >  include/linux/kvm_host.h     |    9 ++++++---
> > >  virt/kvm/kvm_main.c          |   10 ++++++----
> > >  6 files changed, 18 insertions(+), 10 deletions(-)
> > > 
> > > diff --git a/arch/ia64/kvm/kvm-ia64.c b/arch/ia64/kvm/kvm-ia64.c
> > > index 012e5dd..96401b5 100644
> > > --- a/arch/ia64/kvm/kvm-ia64.c
> > > +++ b/arch/ia64/kvm/kvm-ia64.c
> > > @@ -1836,7 +1836,7 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
> > >  
> > >  	memslot = id_to_memslot(kvm->memslots, log->slot);
> > >  	r = -ENOENT;
> > > -	if (!memslot->dirty_bitmap)
> > > +	if (!memslots || !memslot->dirty_bitmap)
> > >  		goto out;
> > >  
> > >  	kvm_ia64_sync_dirty_log(kvm, memslot);
> > > diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> > > index 56067db..0417190 100644
> > > --- a/arch/powerpc/kvm/book3s_hv.c
> > > +++ b/arch/powerpc/kvm/book3s_hv.c
> > > @@ -1267,7 +1267,7 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
> > >  
> > >  	memslot = id_to_memslot(kvm->memslots, log->slot);
> > >  	r = -ENOENT;
> > > -	if (!memslot->dirty_bitmap)
> > > +	if (!memslot || !memslot->dirty_bitmap)
> > >  		goto out;
> > >  
> > >  	n = kvm_dirty_bitmap_bytes(memslot);
> > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > > index 2bb9157..07fdd90 100644
> > > --- a/arch/x86/kvm/vmx.c
> > > +++ b/arch/x86/kvm/vmx.c
> > > @@ -2751,6 +2751,7 @@ static gva_t rmode_tss_base(struct kvm *kvm)
> > >  
> > >  		slots = kvm_memslots(kvm);
> > >  		slot = id_to_memslot(slots, KVM_PRIVATE_MEM_SLOTS);
> > > +		BUG_ON(!slot);
> > >  		base_gfn = slot->base_gfn + slot->npages - 3;
> > >  
> > >  		return base_gfn << PAGE_SHIFT;
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 8765485..53fe9b2 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -3139,9 +3139,11 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
> > >  		goto out;
> > >  
> > >  	memslot = id_to_memslot(kvm->memslots, log->slot);
> > > +	r = -ENOENT;
> > > +	if (!memslot)
> > > +		goto out;
> > >  
> > >  	dirty_bitmap = memslot->dirty_bitmap;
> > > -	r = -ENOENT;
> > >  	if (!dirty_bitmap)
> > >  		goto out;
> > >  
> > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > index 7b3d5c4..1955a4e 100644
> > > --- a/include/linux/kvm_host.h
> > > +++ b/include/linux/kvm_host.h
> > > @@ -313,6 +313,7 @@ struct kvm_irq_routing_table {};
> > >   * to get the memslot by its id.
> > >   */
> > >  struct kvm_memslots {
> > > +	int nmemslots;
> > >  	u64 generation;
> > >  	struct kvm_memory_slot memslots[KVM_MEM_SLOTS_NUM];
> > >  };
> > > @@ -397,7 +398,7 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
> > >  
> > >  #define kvm_for_each_memslot(memslot, slots)	\
> > >  	for (memslot = &slots->memslots[0];	\
> > > -	      memslot < slots->memslots + KVM_MEM_SLOTS_NUM && memslot->npages;\
> > > +	      memslot < slots->memslots + slots->nmemslots && memslot->npages;\
> > >  		memslot++)
> > >  
> > >  int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id);
> > > @@ -424,10 +425,12 @@ static inline struct kvm_memslots *kvm_memslots(struct kvm *kvm)
> > >  static inline struct kvm_memory_slot *
> > >  id_to_memslot(struct kvm_memslots *slots, int id)
> > >  {
> > > -	int index = slots->memslots[id].id_to_index;
> > >  	struct kvm_memory_slot *slot;
> > >  
> > > -	slot = &slots->memslots[index];
> > > +	if (id >= slots->nmemslots)
> > > +		return NULL;
> > > +
> > > +	slot = &slots->memslots[slots->memslots[id].id_to_index];
> > >  
> > >  	WARN_ON(slot->id != id);
> > >  	return slot;
> > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > index 3ce2664..ebd3960 100644
> > > --- a/virt/kvm/kvm_main.c
> > > +++ b/virt/kvm/kvm_main.c
> > > @@ -444,7 +444,9 @@ static void kvm_init_memslots_id(struct kvm *kvm)
> > >  	int i;
> > >  	struct kvm_memslots *slots = kvm->memslots;
> > >  
> > > -	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
> > > +	slots->nmemslots = KVM_MEM_SLOTS_NUM;
> > > +
> > > +	for (i = 0; i < kvm->memslots->nmemslots; i++)
> > >  		slots->memslots[i].id_to_index = slots->memslots[i].id = i;
> > >  }
> > >  
> > > @@ -658,10 +660,10 @@ static void sort_memslots(struct kvm_memslots *slots)
> > >  {
> > >  	int i;
> > >  
> > > -	sort(slots->memslots, KVM_MEM_SLOTS_NUM,
> > > +	sort(slots->memslots, slots->nmemslots,
> > >  	      sizeof(struct kvm_memory_slot), cmp_memslot, NULL);
> > >  
> > > -	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
> > > +	for (i = 0; i < slots->nmemslots; i++)
> > >  		slots->memslots[slots->memslots[i].id].id_to_index = i;
> > >  }
> > >  
> > > @@ -898,7 +900,7 @@ int kvm_get_dirty_log(struct kvm *kvm,
> > >  
> > >  	memslot = id_to_memslot(kvm->memslots, log->slot);
> > >  	r = -ENOENT;
> > > -	if (!memslot->dirty_bitmap)
> > > +	if (!memslot || !memslot->dirty_bitmap)
> > >  		goto out;
> > >  
> > >  	n = kvm_dirty_bitmap_bytes(memslot);
> > 
> > I suppose this should be checked earlier, not at id_to_memslot time.
> > eg for kvm_get_dirty_log at 
> > 
> >         r = -EINVAL;
> >         if (log->slot >= KVM_MEMORY_SLOTS)
> >                 goto out;
> > 
> > time
> 
> id_to_memslot seems like a good place to catch all the users since
> that's the only way to get a slot from a slot id after the array is
> sorted.  We need to check both is the slot in bounds (EINVAL), but also
> is it allocated (ENOENT).  id_to_memslot could both of these if we
> wanted to switch it to ERR_PTR.  Thanks,
> 
> Alex

There should never be a reference to a slot out of bounds by KVM itself
(BUG_ON). Only userspace can attempt a reference to such slot.