linux-kernel - Re: CVE-2021-46978: KVM: nVMX: Always make an attempt to map eVMCS after migration

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Date: Thu, 29 Feb 2024 09:08:42 +0100
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Greg KH <gregkh@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>
Cc: cve@...nel.org, linux-kernel@...r.kernel.org, KVM list
 <kvm@...r.kernel.org>
Subject: Re: CVE-2021-46978: KVM: nVMX: Always make an attempt to map eVMCS
 after migration

Greg KH <gregkh@...nel.org> writes:

> On Wed, Feb 28, 2024 at 11:09:50PM +0100, Paolo Bonzini wrote:
>> On 2/28/24 09:14, Greg Kroah-Hartman wrote:
>> > From: gregkh@...nel.org
>> > 
>> > Description
>> > ===========
>> > 
>> > In the Linux kernel, the following vulnerability has been resolved:
>> > 
>> > KVM: nVMX: Always make an attempt to map eVMCS after migration
>> 
>> How does this break the confidentiality, integrity or availability of the
>> host kernel?  It's a fix for a failure to restart the guest after migration.
>> Vitaly can confirm.
>
> It's a fix for the availability of the guest kernel, which now can not
> boot properly, right?  That's why this was selected.  If this is not
> correct, I will be glad to revoke this.
>

To be precise, this issue is about guest's behavior post-migration and
not booting. Also, it should be noted that "Enlightened VMCS" feature is
normally not used for Linux guests on KVM so the "guest kernel" is
actually Windows kernel (or Hyper-V) :-)

Personally, I don't see how this particular issue differs from other KVM
hypervisor bugs. I.e. when hypervisor misbehaves, the guest will likely
suffer and in many cases "suffer" means crash. What *is* important is
who can trigger hypervisor's misbehavior. In case it is guest triggered
(and especially if triggered from CPL!=0), security implications are
possible. In the even worse case when such guest's actions can cause
issues in the host's kernel, the presence of a vulnerability is almost
certain.

Migration is (normally) not guest triggered, it's a deliberate action on
the host.

-- 
Vitaly