Re: [PATCH 01/11] KVM: MMU: avoid pte_list_desc run out in kvm_mmu_pte_write

Avi Kivity
2011-07-27T09:00:28Z

On 07/26/2011 02:25 PM, Xiao Guangrong wrote:
> kvm_mmu_pte_write is unsafe since we need to alloc pte_list_desc in the
> function when spte is prefetched, unfortunately, we can not know how many
> spte need to be prefetched on this path, that means we can use out of the
> free pte_list_desc object in the cache, and BUG_ON() is triggered, also some
> path does not fill the cache, such as INS instruction emulated that does not
> trigger page fault
>
>
> void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
> const u8 *new, int bytes,
> bool guest_initiated)
> @@ -3583,6 +3596,7 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
> break;
> }
>
> + mmu_topup_memory_caches(vcpu);

Please add a comment here describing why it's okay to ignore the error
return.

> spin_lock(&vcpu->kvm->mmu_lock);
> if (atomic_read(&vcpu->kvm->arch.invlpg_counter) != invlpg_counter)
> gentry = 0;
> @@ -3653,7 +3667,7 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
> mmu_page_zap_pte(vcpu->kvm, sp, spte);
> if (gentry&&
> !((sp->role.word ^ vcpu->arch.mmu.base_role.word)
> - & mask.word))
> + & mask.word)&& get_free_pte_list_desc_nr(vcpu))
> mmu_pte_write_new_pte(vcpu, sp, spte,&gentry);

Wow, this bug was here since 2.6.23. Good catch.

Please wrap or rename get_free_pte_list_desc_nr() in rmap_can_add(vcpu)
so it's clearer why we're doing this.

--
error compiling committee.c: too many arguments to function