Re: [PATCH 2/6] KVM MMU: fix kvm_mmu_zap_page() and its calling path

[Avi Kivity <avi@redhat.com>]

On 04/12/2010 11:53 AM, Xiao Guangrong wrote:
>>
>>>            kvm->arch.n_free_mmu_pages = 0;
>>> @@ -1589,7 +1589,8 @@ static void mmu_unshadow(struct kvm *kvm, gfn_t
>>> gfn)
>>>            &&   !sp->role.invalid) {
>>>                pgprintk("%s: zap %lx %x\n",
>>>                     __func__, gfn, sp->role.word);
>>> -            kvm_mmu_zap_page(kvm, sp);
>>> +            if (kvm_mmu_zap_page(kvm, sp))
>>> +                nn = bucket->first;
>>>            }
>>>        }
>>>
>>>        
>> I don't understand why this is needed.
>>      
> There is the code segment in mmu_unshadow():
>
> |hlist_for_each_entry_safe(sp, node, nn, bucket, hash_link) {
> |		if (sp->gfn == gfn&&  !sp->role.direct
> |		&&  !sp->role.invalid) {
> |			pgprintk("%s: zap %lx %x\n",
> |				 __func__, gfn, sp->role.word);
> |			kvm_mmu_zap_page(kvm, sp);
> |		}
> |	}
>
> in the loop, if nn is zapped, hlist_for_each_entry_safe() access nn will
> cause crash. and it's checked in other functions, such as kvm_mmu_zap_all(),
> kvm_mmu_unprotect_page()...
>    

hlist_for_each_entry_safe() is supposed to be be safe against removal of 
the element that is pointed to by the iteration cursor.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.