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


  • To: Jan Beulich <jbeulich@xxxxxxxx>, Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
  • From: Ayan Kumar Halder <ayankuma@xxxxxxx>
  • Date: Wed, 14 Aug 2024 13:33:58 +0100
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  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Penny Zheng <penny.zheng@xxxxxxx>, Wei Chen <wei.chen@xxxxxxx>, sstabellini@xxxxxxxxxx, bertrand.marquis@xxxxxxx, michal.orzel@xxxxxxx, Volodymyr_Babchuk@xxxxxxxx, julien@xxxxxxx
  • Delivery-date: Wed, 14 Aug 2024 12:34:23 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

Hi Jan,

On 14/08/2024 12:35, Jan Beulich wrote:
On 14.08.2024 12:55, Ayan Kumar Halder wrote:
Hi Jan,

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.

- Ayan


Jan



 


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