Re: [PATCH v3] xen: detect uninitialized xenbus in xenbus_init

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.11.2021 06:42, Juergen Gross wrote:
> On 22.11.21 23:16, Stefano Stabellini wrote:
>> From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
>>
>> If the xenstore page hasn't been allocated properly, reading the value
>> of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually return
>> error. Instead, it will succeed and return zero. Instead of attempting
>> to xen_remap a bad guest physical address, detect this condition and
>> return early.
>>
>> Note that although a guest physical address of zero for
>> HVM_PARAM_STORE_PFN is theoretically possible, it is not a good choice
>> and zero has never been validly used in that capacity.
>>
>> Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.
>>
>> For 32-bit Linux, any pfn above ULONG_MAX would get truncated. Pfns
>> above ULONG_MAX should never be passed by the Xen tools to HVM guests
>> anyway, so check for this condition and return early.
>>
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
>> ---
>> Changes in v3:
>> - improve in-code comment
>> - improve check
>>
>> Changes in v2:
>> - add check for ULLONG_MAX (unitialized)
>> - add check for ULONG_MAX #if BITS_PER_LONG == 32 (actual error)
>> - add pr_err error message
>> ---
>>   drivers/xen/xenbus/xenbus_probe.c | 24 ++++++++++++++++++++++++
>>   1 file changed, 24 insertions(+)
>>
>> diff --git a/drivers/xen/xenbus/xenbus_probe.c 
>> b/drivers/xen/xenbus/xenbus_probe.c
>> index 94405bb3829e..d3ca57d48a73 100644
>> --- a/drivers/xen/xenbus/xenbus_probe.c
>> +++ b/drivers/xen/xenbus/xenbus_probe.c
>> @@ -951,6 +951,30 @@ 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.
>> +             *
>> +             * Also recognize the invalid value of INVALID_PFN which is
>> +             * ULLONG_MAX.
> 
> Adjust the comment, e.g. s/ULLONG_MAX/all bits set/ (in the commit
> message, too)?

I also don't think the reference to INVALID_PFN is appropriate here. Afaict
the two aren't the same on 32-bit. Plus I can't even find a constant named
this way in Linux'es include/.

>> +             */
>> +            if (!v || !(v + 1)) {
> 
> For me "if (!v || !~v)" would be more readable, but I don't really feel
> strong here.

Oh, indeed.

Jan