linux-kernel - Re: [PATCH v1 1/1] s390/uv: Panic if the security of the system cannot be guaranteed.

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Message-ID: <7f98cb85-de83-41ea-aaab-d76a22647ccb@linux.ibm.com>
Date: Thu, 1 Aug 2024 15:20:30 +0200
From: Janosch Frank <frankja@...ux.ibm.com>
To: Claudio Imbrenda <imbrenda@...ux.ibm.com>, linux-kernel@...r.kernel.org
Cc: kvm@...r.kernel.org, linux-s390@...r.kernel.org, hca@...ux.ibm.com,
        agordeev@...ux.ibm.com, gor@...ux.ibm.com, borntraeger@...ibm.com,
        svens@...ux.ibm.com, seiden@...ux.ibm.com, nsg@...ux.ibm.com,
        nrb@...ux.ibm.com
Subject: Re: [PATCH v1 1/1] s390/uv: Panic if the security of the system
 cannot be guaranteed.

On 8/1/24 1:25 PM, Claudio Imbrenda wrote:
> The return value uv_set_shared() and uv_remove_shared() (which are
> wrappers around the share() function) is not always checked. The system
> integrity of a protected guest depends on the Share and Unshare UVCs
> being successful. This means that any caller that fails to check the
> return value will compromise the security of the protected guest.
> 
> No code path that would lead to such violation of the security
> guarantees is currently exercised, since all the areas that are shared
> never get unshared during the lifetime of the system. This might
> change and become an issue in the future.

For people wondering what the effects might be, this is the important 
paragraph to read. Fortunately we're currently not unsharing anything.

Claudio already stated that there's no way out of this but I want to 
reiterate on this. The hypervisor has to mess up quite badly to force a 
rc > 0 for the guest. Likewise the guest has to mess up memory 
management to achieve a rc > 0.

The only time where the cause of the rc can be fixed is when the 
hypervisor is malicious and tracks its changes. In all other cases we 
won't know why we ended up with a rc and it makes sense to stop the VM 
before something worse happens.

@Claudio:
The patch subject is a bit non-specific.
How about:
"s390/uv: Panic for set and remove shared access UVC errors"

With that fixed:
Reviewed-by: Janosch Frank <frankja@...ux.ibm.com>