Re: [PATCH v3 2/4] xen: make VMAP only support in MMU system

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.08.2024 14:33, Ayan Kumar Halder wrote:
> On 14/08/2024 12:35, Jan Beulich wrote:
>> On 14.08.2024 12:55, Ayan Kumar Halder wrote:
>>> On 14/08/2024 07:37, Jan Beulich wrote:
>>>> On 13.08.2024 19:13, Ayan Kumar Halder wrote:
>>>>> From: Penny Zheng <penny.zheng@xxxxxxx>
>>>>>
>>>>> Introduced CONFIG_VMAP which is selected by the architectures that use
>>>>> MMU. vm_init() does not do anything if CONFIG_VMAP is not enabled.
>>>>>
>>>>> VMAP is widely used in ALTERNATIVE feature to remap a range of memory
>>>>> with new memory attributes. Since this is highly dependent on virtual
>>>>> address translation, we choose to fold VMAP in MMU system.
>>>>>
>>>>> In this patch, we introduce a new Kconfig CONFIG_HAS_VMAP, and make it
>>>>> only support in MMU system on ARM architecture. And ALTERNATIVE now
>>>>> depends on VMAP.
>>>>>
>>>>> HARDEN_BRANCH_PREDICTOR is now gated on HAS_VMAP as speculative
>>>>> attacks are not possible on non MMU based systems (ie Cortex-R52, R82).
>>>>> See 
>>>>> https://developer.arm.com/Arm%20Security%20Center/Speculative%20Processor%20Vulnerability.
>>>> While I'm not an Arm expert and hence I'm likely missing aspects, I 
>>>> question
>>>> the one (Spectre-BHB) vulnerability there to be sufficient to draw a
>>>> conclusion towards the usefulness of branch hardening. I would advise
>>>> against encoding such a connection in the Kconfig dependencies.
>>> AFAIU, to address 'Spectre' like vulnerabilities 'branch hardening' was
>>> added.
>>>
>>> See https://lore.kernel.org/all/E1fNadD-0000fz-9r@xxxxxxxxxxxxxxxxxxxxxx/
>>>
>>> And from
>>> https://lists.linaro.org/archives/list/linux-stable-mirror@xxxxxxxxxxxxxxxx/message/F4MGL4WT2R7T54NLRDGKFRQNSXF3DZGD/
>>>
>>> Spectre is valid on MMU based systems.
>> Since then various other issues / flavors were found. I've been focusing
>> on the x86 side of things, but I'd be very surprised if some didn't
>> affect other architectures as well.
> 
> We are talking about Arm here as "HARDEN_BRANCH_PREDICTOR" is specific 
> to Arm.
> 
> https://developer.arm.com/Arm%20Security%20Center/Speculative%20Processor%20Vulnerability
>  
> covers all the flavours and it does not include Cortex-R82 or R52.
> 
> It says the following :-
> 
> "Cortex-R cores typically use a closed software stack. In those 
> environments, applications or processes are strictly controlled, and 
> therefore not exploitable"
> 
>> Plus branch hardening can be a pre-
>> cautionary measure, too, I think.
> 
> The first two Arm non MMU cores that we wish to support in the 
> forthcoming series is Cortex-R82 and R52.
> 
> As seen in https://developer.arm.com/documentation/ka005109/latest/, it 
> explicitly states the following about R82
> 
> The Cortex-R82 implements the faulting feature (FEAT_FPAC) but is not 
> vulnerable. The Cortex-R82 behaves differently than vulnerable A-class 
> CPUs when speculatively executing past an instruction that authenticates 
> PAC, and that behavior does not allow the attack software to create the 
> "oracle".
> 
> We can re-enable branch hardening if we know there is a valid non MMU 
> Arm core which is vulnerable.
> 
> Let me know if you are ok with the rationale.

I'm okay with any rationale. As indicated, I'm likely missing (large) parts
of the picture here. The connection between the two Kconfig settings merely
seemed questionable to me, hence the comment.

Jan