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Date:   Wed, 4 Aug 2021 16:23:26 +0200
From:   Joerg Roedel <jroedel@suse.de>
To:     Dan Carpenter <dan.carpenter@oracle.com>
Cc:     kvm@vger.kernel.org
Subject: Re: [bug report] x86/sev: Split up runtime #VC handler for correct
 state tracking
Message-ID: <YQqi3lKFmR7g/kIl@suse.de>
References: <20210804095725.GA8011@kili>
 <YQqKS7ayK1qkmNzv@suse.de>
 <20210804125834.GF22532@kadam>
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On Wed, Aug 04, 2021 at 03:58:34PM +0300, Dan Carpenter wrote:
> exc_page_fault() <-- called with preempt disabled
> --> kvm_handle_async_pf()
>     --> __kvm_handle_async_pf()
>         --> kvm_async_pf_task_wait_schedule() calls schedule().

This call path can not be taken in the page-fault handler when called
from the #VC handler. To take this path the host needs to inject an
async page-fault, especially setting async pf flags, without injecting a
page-fault exception on its own ... and when the #VC handler is running.
KVM is not doing that.

Okay, the hypervisor can be malicious, but otherwise this can't happen.
To mitigate a malicious hypervisor threat here it might be a solution to
not call the page-fault handler directly from the #VC handler and let it
re-fault after the #VC handler returned.

Regards,

	Joerg