linux-kernel - Re: [PATCH 02/10] kvm: Check userspace

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Date:	Fri, 7 Dec 2012 13:17:25 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	mtosatti@...hat.com, gleb@...hat.com, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH 02/10] kvm: Check userspace_addr when modifying a memory
 slot

On Thu, Dec 06, 2012 at 03:20:37PM -0700, Alex Williamson wrote:
> The API documents that only flags and guest physical memory space can
> be modified on an existing slot, but we don't enforce that the
> userspace address cannot be modified.  Instead we just ignore it.
> This means that a user may think they've successfully moved both the
> guest and user addresses, when in fact only the guest address changed.
> Check and error instead.
> 
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
>  virt/kvm/kvm_main.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e426704..93213e1 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -779,13 +779,19 @@ int __kvm_set_memory_region(struct kvm *kvm,
>  
>  	r = -ENOMEM;
>  
> -	/* Allocate if a slot is being created */
> +	/*
> +	 * Allocate if a slot is being created.  If modifying a slot,
> +	 * the userspace_addr cannot change.
> +	 */
>  	if (!old.npages) {
>  		new.user_alloc = user_alloc;
>  		new.userspace_addr = mem->userspace_addr;
>  
>  		if (kvm_arch_create_memslot(&new, npages))
>  			goto out_free;
> +	} else if (mem->userspace_addr != old.userspace_addr) {
> +		r = -EINVAL;
> +		goto out_free;
>  	}
>  
>  	/* Allocate page dirty bitmap if needed */
> 

hmmm...does this mean that on a 'destroy', where npages is 0, the user
has to set up userspace_addr correctly? If so, that would appear to be
an unwanted change in semantics here.

Thanks,

-Jason
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