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Subject: Re: [PATCH 2/4] KVM: x86: Introduce paravirt feature CR0/CR4 pinning
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     "Andersen, John" <john.s.andersen@intel.com>, corbet@lwn.net,
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Message-ID: <19b97891-bbb0-1061-5971-549a386f7cfb@intel.com>
Date:   Tue, 7 Jul 2020 14:48:29 -0700
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On 7/7/20 2:12 PM, Sean Christopherson wrote:
>>>> Let's say Intel loses its marbles and adds a CR4 bit that lets userspace
>>>> write to kernel memory.  Linux won't set it, but an attacker would go
>>>> after it, first thing.
> That's an orthogonal to pinning.  KVM never lets the guest set CR4 bits that
> are unknown to KVM.  Supporting CR4.NO_MARBLES would require an explicit KVM
> change to allow it to be set by the guest, and would also require a userspace
> VMM to expose NO_MARBLES to the guest.
> 
> That being said, this series should supporting pinning as much as possible,
> i.e. if the bit can be exposed to the guest and doesn't require special
> handling in KVM, allow it to be pinned.  E.g. TS is a special case because
> pinning would require additional emulator support and IMO isn't interesting
> enough to justify the extra complexity.  At a glance, I don't see anything
> that would prevent pinning FSGSBASE.

Thanks for filling in the KVM picture.

If we're supporting as much pinning as possible, can we also add
something to make it inconvenient for someone to both make a CR4 bit
known to KVM *and* ignore the pinning aspects?

We should really make folks think about it.  Something like:

#define KVM_CR4_KNOWN 0xff
#define KVM_CR4_PIN_ALLOWED 0xf0
#define KVM_CR4_PIN_NOT_ALLOWED 0x0f

BUILD_BUG_ON(KVM_CR4_KNOWN !=
             (KVM_CR4_PIN_ALLOWED|KVM_CR4_PIN_NOT_ALLOWED));

So someone *MUST* make an active declaration about new bits being pinned
or not?