linux-kernel - Re: [PATCH] xen: detect uninitialized xenbus in xenbus

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Date:   Thu, 18 Nov 2021 09:40:42 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     boris.ostrovsky@...cle.com, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano.stabellini@...inx.com>,
        stable@...r.kernel.org, jgross@...e.com
Subject: Re: [PATCH] xen: detect uninitialized xenbus in xenbus_init

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