Re: [PATCH v2 1/5] Drivers: hv: vmbus: Leak pages if set_memory_encrypted() fails

[Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>]

Hi,

On 3/11/24 9:15 AM, mhkelley58@gmail.com wrote:
> From: Rick Edgecombe <rick.p.edgecombe@intel.com>
>
> In CoCo VMs it is possible for the untrusted host to cause
> set_memory_encrypted() or set_memory_decrypted() to fail such that an
> error is returned and the resulting memory is shared. Callers need to
> take care to handle these errors to avoid returning decrypted (shared)
> memory to the page allocator, which could lead to functional or security
> issues.
>
> VMBus code could free decrypted pages if set_memory_encrypted()/decrypted()
> fails. Leak the pages if this happens.
>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
> ---
LGTM

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
>  drivers/hv/connection.c | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
> index 3cabeeabb1ca..f001ae880e1d 100644
> --- a/drivers/hv/connection.c
> +++ b/drivers/hv/connection.c
> @@ -237,8 +237,17 @@ int vmbus_connect(void)
>  				vmbus_connection.monitor_pages[0], 1);
>  	ret |= set_memory_decrypted((unsigned long)
>  				vmbus_connection.monitor_pages[1], 1);
> -	if (ret)
> +	if (ret) {
> +		/*
> +		 * If set_memory_decrypted() fails, the encryption state
> +		 * of the memory is unknown. So leak the memory instead
> +		 * of risking returning decrypted memory to the free list.
> +		 * For simplicity, always handle both pages the same.
> +		 */
> +		vmbus_connection.monitor_pages[0] = NULL;
> +		vmbus_connection.monitor_pages[1] = NULL;
>  		goto cleanup;
> +	}
>  
>  	/*
>  	 * Set_memory_decrypted() will change the memory contents if
> @@ -337,13 +346,19 @@ void vmbus_disconnect(void)
>  		vmbus_connection.int_page = NULL;
>  	}
>  
> -	set_memory_encrypted((unsigned long)vmbus_connection.monitor_pages[0], 1);
> -	set_memory_encrypted((unsigned long)vmbus_connection.monitor_pages[1], 1);
> +	if (vmbus_connection.monitor_pages[0]) {
> +		if (!set_memory_encrypted(
> +			(unsigned long)vmbus_connection.monitor_pages[0], 1))
> +			hv_free_hyperv_page(vmbus_connection.monitor_pages[0]);
> +		vmbus_connection.monitor_pages[0] = NULL;
> +	}
>  
> -	hv_free_hyperv_page(vmbus_connection.monitor_pages[0]);
> -	hv_free_hyperv_page(vmbus_connection.monitor_pages[1]);
> -	vmbus_connection.monitor_pages[0] = NULL;
> -	vmbus_connection.monitor_pages[1] = NULL;
> +	if (vmbus_connection.monitor_pages[1]) {
> +		if (!set_memory_encrypted(
> +			(unsigned long)vmbus_connection.monitor_pages[1], 1))
> +			hv_free_hyperv_page(vmbus_connection.monitor_pages[1]);
> +		vmbus_connection.monitor_pages[1] = NULL;
> +	}
>  }
>  
>  /*

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer