Date:   Fri, 19 Nov 2021 06:16:20 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     Jan Beulich <jbeulich@...e.com>, boris.ostrovsky@...cle.com,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano.stabellini@...inx.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] xen: detect uninitialized xenbus in xenbus_init

On 18.11.21 22:00, Stefano Stabellini wrote:
> On Thu, 18 Nov 2021, Juergen Gross wrote:
>> On 18.11.21 09:47, Jan Beulich wrote:
>>> 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).
> 
> Sure, I can do that
> 
> 
>>>>> +			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
>>
>> I could imagine someone wanting to run a guest without Xenstore access,
>> which BTW will happen in case of a guest created by the hypervisor at
>> boot time.
>>
>>> 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.
>>
>> Yes, but this would still be a major tools problem which probably
>> would render the whole guest rather unusable.
> 
> First let's distinguish between an error due to "hvm_param not
> initialized" and an error due to more serious conditions, such as "pfn
> above max".
> 
> "hvm_param not initialized" could mean v == 0 (as it would be today) or
> v == ~0UL (if we change Xen to initialize all hvm_param to ~0UL). I
> don't think we want to panic in these cases as they are not actually
> true erroneous configurations. We should just stop trying to initialize
> xenstore and continue with the rest.
> 
> 
> The "pfn above max" case could happen if v is greater than the max pfn.
> This is a true error in the configuration because the toolstack should
> know that the guest is 32-bit so it should give it a pfn that the guest

I don't think so. All x86 PVH/HVM guests start booting in 32-bit mode.

> is able to use. As Jan wrote in another email, for 32-bit the actual
> limit depends on the physical address bits but actually Linux has never
> been able to cope with a pfn > ULONG_MAX on 32-bit because xen_store_gfn
> is defined as unsigned long. So Linux 32-bit has been truncating
> HVM_PARAM_STORE_PFN all along.

The question is whether the number of physical address bits as presented
to the guest is always >= 44. If not the actual limit is less than
ULONG_MAX. Other than that you are right: a PFN larger than a 32-bit
ULONG_MAX will be truncated by a 32-bit guest.

> There is also an argument that depending on kconfig Linux 32-bit might
> only be able to handle addresses < 4G, so I don't think the toolstack
> can assume that a 32-bit guest is able to cope with HVM_PARAM_STORE_PFN
>> ULONG_MAX.  If Linux is 32-bit and HVM_PARAM_STORE_PFN > ULONG_MAX,
> even if HVM_PARAM_STORE_PFN < address_bits_max I think it would be fair
> to still consider it an error, but I can see it could be argued either
> way. Certainly if HVM_PARAM_STORE_PFN > address_bits_max is an error.

Right. The tools should NEVER put the frame above 4G for a non-PV guest.

> In any case, I think it is still better for Linux to stop trying to
> initialize Xenstore but continue with the rest because there is a bunch
> of other useful things Linux can do without it. Panic should only be the
> last resort if there is nothing else to do. In this case we haven't even
> initialized the service and the service is not essential, at least it is
> not essential in certain ARM setups.
> 
> 
> So in conclusion, I think this patch should:
> - if v == 0 return error (uninitialized)
> - if v == ~0ULL (INVALID_PFN) return error (uinitialized)
> - if v >= ~0UL (32-bit) return error (even if this case could be made to
>    work for v < max_address_bits depending on kconfig)
> 
> Which leads to something like:
> 
>          /* uninitialized */
> 		if (v == 0 || v == ~0ULL) {
> 			err = -ENOENT;
> 			goto out_error;
> 		}
>          /*
>           * Avoid truncation on 32-bit.
>           * TODO: handle addresses >= 4G
>           */
>          if ( v >= ~0UL ) {
>              err = -EINVAL;
>              goto out_error;

I think at least in this case a pr_err("...") should be added.

Silent failure is not nice.


Juergen

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