Re: [PATCH] xen/arm64: Zero the top 32 bits of gp registers on entry...

[Michal Orzel <michal.orzel@xxxxxxx>]

Hi Julien, Jan

On 08.12.2021 10:55, Julien Grall wrote:
> Hi,
> 
> On 08/12/2021 07:20, Jan Beulich wrote:
>> On 07.12.2021 20:11, Julien Grall wrote:
>>>
>>>
>>> On 07/12/2021 08:37, Michal Orzel wrote:
>>>> Hi Julien,
>>>
>>> Hi,
>>>
>>>> On 06.12.2021 16:29, Julien Grall wrote:
>>>>> Hi,
>>>>>
>>>>> On 06/12/2021 14:20, Michal Orzel wrote:
>>>>>> to hypervisor when switching to AArch32 state.
>>>>>>
>>>> I will change to "from AArch32 state".
>>>>>> According to section D1.20.2 of Arm Arm(DDI 0487A.j):
>>>>>> "If the general-purpose register was accessible from AArch32 state the
>>>>>> upper 32 bits either become zero, or hold the value that the same
>>>>>> architectural register held before any AArch32 execution.
>>>>>> The choice between these two options is IMPLEMENTATIONDEFINED"
>>>>>
>>>>> Typo: Missing space between IMPLEMENTATION and DEFINED.
>>>>>
>>>> Ok.
>>>>>>
>>>>>> Currently Xen does not ensure that the top 32 bits are zeroed and this
>>>>>> needs to be fixed.
>>>>>
>>>>> Can you outline why this is a problem and why we need to protect? IIRC, 
>>>>> the main concern is Xen may misinterpret what the guest requested but we 
>>>>> are not concerned about Xen using wrong value.
>>>>>
>>>> I would say:
>>>> "
>>>> The reason why this is a problem is that there are places in Xen where we 
>>>> assume that top 32bits are zero for AArch32 guests.
>>>> If they are not, this can lead to misinterpretation of Xen regarding what 
>>>> the guest requested.
>>>> For example hypercalls returning an error encoded in a signed long like 
>>>> do_sched_op, do_hmv_op, do_memory_op would return -ENOSYS
>>>> if the command passed as the first argument was clobbered,
>>>> "
>>>>>>
>>>>>> Fix this bug by zeroing the upper 32 bits of these registers on an
>>>>>> entry to hypervisor when switching to AArch32 state.
>>>>>>
>>>>>> Set default value of parameter compat of macro entry to 0 (AArch64 mode
>>>>>> as we are on 64-bit hypervisor) to avoid checking if parameter is blank
>>>>>> when not passed.
>>>>>
>>>>> Which error do you see otherwise? Is it a compilation error?
>>>>>
>>>> Yes, this is a compilation error. The errors appear at each line when 
>>>> "entry" is called without passing value for "compat".
>>>> So basically in all the places where entry is called with hyp=1.
>>>> When taking the current patch and removing default value for compat you 
>>>> will get:
>>>> ```
>>>> entry.S:254: Error: ".endif" without ".if"
>>>> entry.S:258: Error: symbol `.if' is already defined
>>>> entry.S:258: Error: ".endif" without ".if"
>>>> entry.S:262: Error: symbol `.if' is already defined
>>>> entry.S:262: Error: ".endif" without ".if"
>>>> entry.S:266: Error: symbol `.if' is already defined
>>>> entry.S:266: Error: ".endif" without ".if"
>>>> entry.S:278: Error: symbol `.if' is already defined
>>>> entry.S:278: Error: ".endif" without ".if"
>>>> entry.S:292: Error: symbol `.if' is already defined
>>>> entry.S:292: Error: ".endif" without ".if"
>>>> entry.S:317: Error: symbol `.if' is already defined
>>>> entry.S:317: Error: ".endif" without ".if"
>>>> ```
>>>
>>> Thanks for input. I am concerned with your suggested approach (or using
>>> .if 0\compat as suggested by Jan) because they allow the caller to not
>>> properly specify compat when hyp=0. The risk here is we may generate the
>>> wrong entry.
>>>
>>> compat should need to be specified when hyp=1 as we will always run in
>>> aarch64 mode. So could we protect this code with hyp=0?
>>
>> Since my suggestion was only to avoid the need for specifying a default
>> for the parameter (which you didn't seem to be happy about), it would
>> then merely extend to
>>
>> .if !0\hyp && 0\compat
> Isn't it effectively the same as setting a default value?
> 
> The reason we seem to get away is because other part of the macro (e.g. 
> entry_guest) will need compat to be valid.
> 
> But that seems pretty fragile to me. So I would prefer if the new code it 
> added within a macro that is only called when hyp==0.
> 
So you would like to have a macro that is called if hyp=0 (which means compat 
had to be passed) and inside this macro additional check if compat is 1?
> Cheers,
> 
>>
>> or something along those lines.
>>
>> Jan
>>
> 
So when it comes to zeroing the top 32bits by pushing zero to higher halves of 
stack slots I would do in a loop:
stp wzr, wzr, [sp #8 * 0]
stp wzr, wzr, [sp #8 * 1]
...

FWIK this would store 0 in the upper addresses. Am I correct?

Cheers,
Michal