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

[Jan Beulich <jbeulich@suse.com>]

On 18.11.2021 06:32, Juergen Gross wrote:
> On 18.11.21 03:37, Stefano Stabellini wrote:
>> --- a/drivers/xen/xenbus/xenbus_probe.c
>> +++ b/drivers/xen/xenbus/xenbus_probe.c
>> @@ -951,6 +951,28 @@ static int __init xenbus_init(void)
>>   		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
>>   		if (err)
>>   			goto out_error;
>> +		/*
>> +		 * Return error on an invalid value.
>> +		 *
>> +		 * 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) {
> 
> Make this "if (v == ULONG_MAX || v== 0)" instead?
> This would result in the same err on a new and an old hypervisor
> (assuming we switch the hypervisor to init params with ~0UL).
> 
>> +			err = -ENOENT;
>> +			goto out_error;
>> +		}
>> +		/*
>> +		 * ULONG_MAX is invalid on 64-bit because is INVALID_PFN.
>> +		 * On 32-bit return error to avoid truncation.
>> +		 */
>> +		if (v >= ULONG_MAX) {
>> +			err = -EINVAL;
>> +			goto out_error;
>> +		}
> 
> Does it make sense to continue the system running in case of
> truncation? This would be a 32-bit guest with more than 16TB of RAM
> and the Xen tools decided to place the Xenstore ring page above the
> 16TB boundary. This is a completely insane scenario IMO.
> 
> A proper panic() in this case would make diagnosis of that much
> easier (me having doubts that this will ever be hit, though).

While I agree panic() may be an option here (albeit I'm not sure why
that would be better than trying to cope with 0 and hence without
xenbus), I'd like to point out that the amount of RAM assigned to a
guest is unrelated to the choice of GFNs for the various "magic"
items.

Jan