Re: [PATCH v5 09/19] KVM:x86: Make guest supervisor states as non-XSAVE managed

["Yang, Weijiang" <weijiang.yang@intel.com>]

On 8/10/2023 11:15 PM, Paolo Bonzini wrote:
> On 8/10/23 16:29, Dave Hansen wrote:
>> On 8/10/23 02:29, Yang, Weijiang wrote:
>> ...
>>> When KVM enumerates shadow stack support for guest in CPUID(0x7,
>>> 0).ECX[bit7], architecturally it claims both SS user and supervisor
>>> mode are supported. Although the latter is not supported in Linux,
>>> but in virtualization world, the guest OS could be non-Linux system,
>>> so KVM supervisor state support is necessary in this case.
>>
>> What actual OSes need this support?
>
> I think Xen could use it when running nested.  But KVM cannot expose support for CET in CPUID, and at the same time fake support for MSR_IA32_PL{0,1,2}_SSP (e.g. inject a #GP if it's ever written to a nonzero value).
>
> I suppose we could invent our own paravirtualized CPUID bit for "supervisor IBT works but supervisor SHSTK doesn't".  Linux could check that but I don't think it's a good idea.
>
> So... do, or do not.  There is no try. :)
>
>>> Two solutions are on the table:
>>> 1) Enable CET supervisor support in Linux kernel like user mode support.
>>
>> We _will_ do this eventually, but not until FRED is merged.  The core
>> kernel also probably won't be managing the MSRs on non-FRED hardware.
>>
>> I think what you're really talking about here is that the kernel would
>> enable CET_S XSAVE state management so that CET_S state could be managed
>> by the core kernel's FPU code.
>
> Yes, I understand it that way too.

Sorry for confusion, I missed the word "state" here.

>> That is, frankly, *NOT* like the user mode support at all.
>
> I agree.
>
>>> 2) Enable support in KVM domain.
>>>
>>> Problem:
>>> The Pros/Cons for each solution(my individual thoughts):
>>> In kernel solution:
>>> Pros:
>>> - Avoid saving/restoring 3 supervisor MSRs(PL{0,1,2}_SSP) at vCPU
>>>    execution path.
>>> - Easy for KVM to manage guest CET xstate bits for guest.
>>> Cons:
>>> - Unnecessary supervisor state xsaves/xrstors operation for non-vCPU
>>>    thread.
>>
>> What operations would be unnecessary exactly?
>
> Saving/restoring PL0/1/2_SSP when switching from one usermode task's fpstate to another.
>
>>> KVM solution:
>>> Pros:
>>> - Not touch current kernel FPU management framework and logic.
>>> - No extra space and operation for non-vCPU thread.
>>> Cons:
>>> - Manually saving/restoring 3 supervisor MSRs is a performance burden to
>>>    KVM.
>>> - It looks more like a hack method for KVM, and some handling logic
>>>    seems a bit awkward.
>>
>> In a perfect world, we'd just allocate space for CET_S in the KVM
>> fpstates.  The core kernel fpstates would have
>> XSTATE_BV[13]==XCOMP_BV[13]==0.  An XRSTOR of the core kernel fpstates
>> would just set CET_S to its init state.
>
> Yep.  I don't think it's a lot of work to implement.  The basic idea as you point out below is something like
>
> #define XFEATURE_MASK_USER_DYNAMIC XFEATURE_MASK_XTILE_DATA
> #define XFEATURE_MASK_USER_OPTIONAL \
>     (XFEATURE_MASK_DYNAMIC | XFEATURE_MASK_CET_KERNEL)
>
> where XFEATURE_MASK_USER_DYNAMIC is used for xfd-related tasks (including the ARCH_GET_XCOMP_SUPP arch_prctl) but everything else uses XFEATURE_MASK_USER_OPTIONAL.
>
> KVM would enable the feature by hand when allocating the guest fpstate. Disabled features would be cleared from EDX:EAX when calling XSAVE/XSAVEC/XSAVES.

OK, I'll move ahead in that direction.

>> But I suspect that would be too much work to implement in practice.  It
>> would be akin to a new lesser kind of dynamic xstate, one that didn't
>> interact with XFD and *NEVER* gets allocated in the core kernel
>> fpstates, even on demand.
>>
>> I want to hear more about who is going to use CET_S state under KVM in
>> practice.  I don't want to touch it if this is some kind of purely
>> academic exercise.  But it's also silly to hack some kind of temporary
>> solution into KVM that we'll rip out in a year when real supervisor
>> shadow stack support comes along.
>>
>> If it's actually necessary, we should probably just eat the 24 bytes in
>> the fpstates, flip the bit in IA32_XSS and move on.  There shouldn't be
>> any other meaningful impact to the core kernel.
>
> If that's good to you, why not.

Thanks to all of you for quickly helping out!

> Paolo
>