Subject: Re: [PATCH kernel v4 08/10] KVM: PPC: Separate TCE validation from update

From: David Gibson <[email-masked]>
Date: Thu, 9 Feb 2017 14:51:51 +1100

On Tue, Feb 07, 2017 at 06:17:09PM +1100, Alexey Kardashevskiy wrote:
> For the emulated devices it does not matter much if we get a broken TCE
> half way handling a TCE list but for VFIO it will matter as it has
> more chances to fail so we try to do our best and check as much as we
> can before proceeding.
> 
> This separates a guest view table update from validation. No change in
> behavior is expected.
> 
> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> ---
>  arch/powerpc/kvm/book3s_64_vio.c    | 8 ++++++++
>  arch/powerpc/kvm/book3s_64_vio_hv.c | 8 ++++++--
>  2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
> index 15df8ae627d9..9a7b7fca5e84 100644
> --- a/arch/powerpc/kvm/book3s_64_vio.c
> +++ b/arch/powerpc/kvm/book3s_64_vio.c
> @@ -282,6 +282,14 @@ long kvmppc_h_put_tce_indirect(struct kvm_vcpu *vcpu,
>  		ret = kvmppc_tce_validate(stt, tce);
>  		if (ret != H_SUCCESS)
>  			goto unlock_exit;
> +	}
> +
> +	for (i = 0; i < npages; ++i) {
> +		if (get_user(tce, tces + i)) {
> +			ret = H_TOO_HARD;
> +			goto unlock_exit;
> +		}
> +		tce = be64_to_cpu(tce);

This doesn't look safe.  The contents of user memory could change
between the two get_user()s, meaning that you're no longer guaranteed
a TCE loaded into kernel has been validated at all.

I think you need to either:

    a) Make sure things safe against a bad TCE being loaded into a TCE
    table and move all validation to where the TCE is used, rather
    than loaded

or
    b) Copy the whole set of indirect entries to a temporary in-kernel
       buffer, then validate, then load into the actual TCE table.

>  
>  		kvmppc_tce_put(stt, entry + i, tce);
>  	}
> diff --git a/arch/powerpc/kvm/book3s_64_vio_hv.c b/arch/powerpc/kvm/book3s_64_vio_hv.c
> index 918af76ab2b6..f8a54b7c788e 100644
> --- a/arch/powerpc/kvm/book3s_64_vio_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_vio_hv.c
> @@ -237,7 +237,7 @@ long kvmppc_rm_h_put_tce_indirect(struct kvm_vcpu *vcpu,
>  {
>  	struct kvmppc_spapr_tce_table *stt;
>  	long i, ret = H_SUCCESS;
> -	unsigned long tces, entry, ua = 0;
> +	unsigned long tces, entry, tce, ua = 0;
>  	unsigned long *rmap = NULL;
>  
>  	stt = kvmppc_find_table(vcpu->kvm, liobn);
> @@ -279,11 +279,15 @@ long kvmppc_rm_h_put_tce_indirect(struct kvm_vcpu *vcpu,
>  	}
>  
>  	for (i = 0; i < npages; ++i) {
> -		unsigned long tce = be64_to_cpu(((u64 *)tces)[i]);
> +		tce = be64_to_cpu(((u64 *)tces)[i]);
>  
>  		ret = kvmppc_tce_validate(stt, tce);
>  		if (ret != H_SUCCESS)
>  			goto unlock_exit;
> +	}
> +
> +	for (i = 0; i < npages; ++i) {
> +		tce = be64_to_cpu(((u64 *)tces)[i]);

Same problem here.

>  
>  		kvmppc_tce_put(stt, entry + i, tce);
>  	}

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson