Re: [PATCH v2 6/8] KVM: Fix multiple races in gfn=>pfn cache refresh

[Sean Christopherson <seanjc@google.com>]

On Wed, Apr 27, 2022, Sean Christopherson wrote:
> Finally, the refresh logic doesn't protect against concurrent refreshes
> with different GPAs (which may or may not be a desired use case, but its
> allowed in the code), nor does it protect against a false negative on the
> memslot generation.  If the first refresh sees a stale memslot generation,
> it will refresh the hva and generation before moving on to the hva=>pfn
> translation.  If it then drops gpc->lock, a different user can come along,
> acquire gpc->lock, see that the memslot generation is fresh, and skip
> the hva=>pfn update due to the userspace address also matching (because
> it too was updated).  Address this race by adding an "in-progress" flag
> so that the refresh that acquires gpc->lock first runs to completion
> before other users can start their refresh.

...

> @@ -159,10 +249,23 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
>  
>  	write_lock_irq(&gpc->lock);
>  
> +	/*
> +	 * If another task is refreshing the cache, wait for it to complete.
> +	 * There is no guarantee that concurrent refreshes will see the same
> +	 * gpa, memslots generation, etc..., so they must be fully serialized.
> +	 */
> +	while (gpc->refresh_in_progress) {
> +		write_unlock_irq(&gpc->lock);
> +
> +		cond_resched();
> +
> +		write_lock_irq(&gpc->lock);
> +	}
> +	gpc->refresh_in_progress = true;

Adding refresh_in_progress can likely go in a separate patch.  I'll plan on doing
that in a v3 unless it proves to be painful.

> @@ -246,9 +296,26 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
>  	}
>  
>   out:
> +	/*
> +	 * Invalidate the cache and purge the pfn/khva if the refresh failed.
> +	 * Some/all of the uhva, gpa, and memslot generation info may still be
> +	 * valid, leave it as is.
> +	 */
> +	if (ret) {
> +		gpc->valid = false;
> +		gpc->pfn = KVM_PFN_ERR_FAULT;
> +		gpc->khva = NULL;
> +	}
> +
> +	gpc->refresh_in_progress = false;