linux-kernel - Re: [PATCH] KVM: x86/mmu: fix potential races when walking host page table

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Date:   Fri, 29 Apr 2022 11:10:26 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Mingwei Zhang <mizhang@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: fix potential races when walking host page
 table

On 4/29/22 05:17, Mingwei Zhang wrote:
> @@ -2838,11 +2836,7 @@ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
>   	 */
>   	hva = __gfn_to_hva_memslot(slot, gfn);
>   
> -	pte = lookup_address_in_mm(kvm->mm, hva, &level);
> -	if (unlikely(!pte))
> -		return PG_LEVEL_4K;
> -
> -	return level;
> +	return kvm_lookup_address_level_in_mm(kvm, hva);
>   }

The function can be just inlined in host_pfn_mapping_level.

>   int kvm_mmu_max_mapping_level(struct kvm *kvm,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 951d0a78ccdae..61406efe4ea7f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -13044,6 +13044,76 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
>   }
>   EXPORT_SYMBOL_GPL(kvm_sev_es_string_io);
>   
> +/*
> + * Lookup the valid mapping level for a virtual address in the current mm.
> + * Return the level of the mapping if there is present one. Otherwise, always
> + * return PG_LEVEL_NONE.

This is a change in semantics, because host_pfn_mapping_level never 
returned PG_LEVEL_NONE.  Returning PG_LEVEL_4K for a non-present entry 
is safe; if it happens, MMU notifiers will force a retry.  If the 
function is inlined in host_pfn_mapping_level, returning PG_LEVEL_4K 
would allow making the semantic change in a separate patch.

In fact, kvm_mmu_hugepage_adjust will go on and set fault->req_level and 
fault->goal_level to PG_LEVEL_NONE, which is wrong even if it does not 
cause havoc.

> + * Note: the information retrieved may be stale. Use it with causion.

The comment should point out that mmu_notifier_retry make it safe to use 
the stale value---of course this is only true if 
kvm_lookup_address_level_in_mm is used where mmu_notifier_retry is used, 
and might be another point in favor of inlining.

> +	ptep = pte_offset_map(&pmd, address);
> +	pte = ptep_get(ptep);
> +	if (pte_present(pte)) {
> +		pte_unmap(ptep);
> +		level = PG_LEVEL_4K;
> +		goto out;
> +	}
> +	pte_unmap(ptep);

Not needed as long as PG_LEVEL_4K is returned for a non-present PTE.

> +out:
> +	local_irq_restore(flags);
> +	return level;
> +}
> +EXPORT_SYMBOL_GPL(kvm_lookup_address_level_in_mm);

Exporting is not needed.

Thanks for writing the walk code though.  I'll adapt it and integrate 
the patch.

Paolo

>   EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_entry);
>   EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_exit);
>   EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_fast_mmio);
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 588792f003345..f1cdcc8483bd0 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -454,4 +454,6 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
>   			 unsigned int port, void *data,  unsigned int count,
>   			 int in);
>   
> +int kvm_lookup_address_level_in_mm(struct kvm *kvm, unsigned long address);
> +
>   #endif
> 
> base-commit: 2a39d8b39bffdaf1a4223d0d22f07baee154c8f3