Re: [PATCH] xen: detect uninitialized xenbus in xenbus_init

[Jan Beulich]

On 18.11.2021 03:37, Stefano Stabellini wrote:
> On Wed, 17 Nov 2021, Jan Beulich wrote:
>> On 17.11.2021 03:11, Stefano Stabellini wrote:
>>> --- a/drivers/xen/xenbus/xenbus_probe.c
>>> +++ b/drivers/xen/xenbus/xenbus_probe.c
>>> @@ -951,6 +951,18 @@ static int __init xenbus_init(void)
>>>  		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
>>>  		if (err)
>>>  			goto out_error;
>>> +		/*
>>> +		 * Uninitialized hvm_params are zero and return no error.
>>> +		 * Although it is theoretically possible to have
>>> +		 * HVM_PARAM_STORE_PFN set to zero on purpose, in reality it is
>>> +		 * not zero when valid. If zero, it means that Xenstore hasn't
>>> +		 * been properly initialized. Instead of attempting to map a
>>> +		 * wrong guest physical address return error.
>>> +		 */
>>> +		if (v == 0) {
>>> +			err = -ENOENT;
>>> +			goto out_error;
>>> +		}
>>
>> If such a check gets added, then I think known-invalid frame numbers
>> should be covered at even higher a priority than zero.
> 
> Uhm, that's a good point. We could check for 0 and also ULONG_MAX

Why ULONG_MAX? The upper bound is determined by the number of physical
address bits (in a guest: the virtual counterpart thereof). In a 32-bit
environment ULONG_MAX could in principle even represent a valid frame
number.

Jan