Subject: Re: [PATCH v4 16/19] KVM: Ensure validity of memslot with respect to kvm_get_dirty_log()

From: Sean Christopherson <[email-masked]>
Date: Tue, 14 Jan 2020 10:25:07 -0800

On Tue, Dec 24, 2019 at 01:19:30PM -0500, Peter Xu wrote:
> On Tue, Dec 17, 2019 at 12:40:38PM -0800, Sean Christopherson wrote:
> > +int kvm_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log,
> > +		      int *is_dirty, struct kvm_memory_slot **memslot)
> >  {
> >  	struct kvm_memslots *slots;
> > -	struct kvm_memory_slot *memslot;
> >  	int i, as_id, id;
> >  	unsigned long n;
> >  	unsigned long any = 0;
> >  
> > +	*memslot = NULL;
> > +	*is_dirty = 0;
> > +
> >  	as_id = log->slot >> 16;
> >  	id = (u16)log->slot;
> >  	if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_USER_MEM_SLOTS)
> >  		return -EINVAL;
> >  
> >  	slots = __kvm_memslots(kvm, as_id);
> > -	memslot = id_to_memslot(slots, id);
> > -	if (!memslot->dirty_bitmap)
> > +	*memslot = id_to_memslot(slots, id);
> > +	if (!(*memslot)->dirty_bitmap)
> >  		return -ENOENT;
> >  
> > -	n = kvm_dirty_bitmap_bytes(memslot);
> > +	kvm_arch_sync_dirty_log(kvm, *memslot);
> 
> Should this line belong to previous patch?

No.

The previous patch, "KVM: Provide common implementation for generic dirty
log functions", is consolidating the implementation of dirty log functions
for architectures with CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT=y.

This code is being moved from s390's kvm_vm_ioctl_get_dirty_log(), as s390
doesn't select KVM_GENERIC_DIRTYLOG_READ_PROTECT.  It's functionally a nop
as kvm_arch_sync_dirty_log() is empty for PowerPC, the only other arch that
doesn't select KVM_GENERIC_DIRTYLOG_READ_PROTECT.

Arguably, the call to kvm_arch_sync_dirty_log() should be moved in a
separate prep patch.  It can't be a follow-on patch as that would swap the
ordering of kvm_arch_sync_dirty_log() and kvm_dirty_bitmap_bytes(), etc...

My reasoning for not splitting it to a separate patch is that prior to this
patch, the common code and arch specific code are doing separate memslot
lookups via id_to_memslot(), i.e. moving the kvm_arch_sync_dirty_log() call
would operate on a "different" memslot.   It can't actually be a different
memslot because slots_lock is held, it just felt weird.

All that being said, I don't have a strong opinion on moving the call to
kvm_arch_sync_dirty_log() in a separate patch; IIRC, I vascillated between
the two options when writing the code.  If anyone wants it to be a separate
patch I'll happily split it out.

> 
> > +
> > +	n = kvm_dirty_bitmap_bytes(*memslot);
> >  
> >  	for (i = 0; !any && i < n/sizeof(long); ++i)
> > -		any = memslot->dirty_bitmap[i];
> > +		any = (*memslot)->dirty_bitmap[i];
> >  
> > -	if (copy_to_user(log->dirty_bitmap, memslot->dirty_bitmap, n))
> > +	if (copy_to_user(log->dirty_bitmap, (*memslot)->dirty_bitmap, n))
> >  		return -EFAULT;
> >  
> >  	if (any)
> > -- 
> > 2.24.1
> 
> -- 
> Peter Xu
> 

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