linux-kernel - Re: [PATCH 2/4] KVM: x86: Introduce paravirt feature CR0/CR4 pinning

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Date:   Thu, 18 Jun 2020 07:51:10 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Andersen, John" <john.s.andersen@...el.com>
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Subject: Re: [PATCH 2/4] KVM: x86: Introduce paravirt feature CR0/CR4 pinning

On 6/18/20 7:43 AM, Andersen, John wrote:
> On Thu, Jun 18, 2020 at 07:18:09AM -0700, Dave Hansen wrote:
>> On 6/17/20 12:07 PM, John Andersen wrote:
>>> +#define KVM_CR0_PIN_ALLOWED	(X86_CR0_WP)
>>> +#define KVM_CR4_PIN_ALLOWED	(X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP)
>>
>> Why *is* there an allowed set?  Why don't we just allow everything?
>>
>> Shouldn't we also pin any unknown bits?  The CR4.FSGSBASE bit is an
>> example of something that showed up CPUs without Linux knowing about it.
>>  If set, it causes problems.  This set couldn't have helped FSGSBASE
>> because it is not in the allowed set.
>>
>> 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.
> 
> The allowed set came about because there were comments from internal review
> where it was said that allowing the guest to pin TS and MP adds unnecessary
> complexity.

That would have been a great design point to include in the changelog.
 Could you make sure it shows up in future versions.

> Also because KVM always intercepts these bits via the CR0/4_GUEST_HOST_MASK. If
> we allow setting of any bits, then we have to add some infrastructure for
> modifying the mask when pinned bits are updated. I have a patch for that if we
> want to go that route, but it doesn't account for the added complexity
> mentioned above.

Well, we have a current, known issue (FSGSBASE) which shows how dealing
with guest-unknown bits is required.  To me, that overrules complexity
arguments to a large degree.