On Tue, Jan 18, 2022 at 4:07 PM Sean Christopherson [off-list ref] wrote:

On Fri, Jan 14, 2022, Reiji Watanabe wrote:

quoted

The restriction, with which KVM doesn't need to worry about the changes
in the registers after KVM_RUN, could potentially protect or be useful
to protect KVM and simplify future changes/maintenance of the KVM codes
that consumes the values.

That sort of protection is definitely welcome, the previously mentioned CPUID mess
on x86 would have benefit greatly by KVM being restrictive in the past.  That said,
hooking KVM_RUN is likely the wrong way to go about implementing any restrictions.
Running a vCPU is where much of the vCPU's state is explicitly consumed, but it's
all too easy for KVM to implicity/indirectly consume state via a different ioctl(),
e.g. if there are side effects that are visible in other registers, than an update
can also be visible to userspace via KVM_{G,S}ET_{S,}REGS, at which point disallowing
modifying state after KVM_RUN but not after reading/writing regs is arbitrary and
inconsitent.

Thank you for your comments !
I think I understand your concern, and that's a great point.
That's not the case for those pseudo registers though at least for now :)
BTW, is this concern specific to hooking KVM_RUN ? (Wouldn't it be the
same for the option with "if kvm->created_vcpus > 0" ?)

If possible, preventing modification if kvm->created_vcpus > 0 is ideal as it's
a relatively common pattern in KVM, and provides a clear boundary to userpace
regarding what is/isn't allowed.

Yes, I agree that would be better in general.  For (pseudo) registers,
I would think preventing modification if kvm->created_vcpus > 0 might
not be a very good option for KVM/ARM though considering usage of
KVM_GET_REG_LIST and KVM_{G,S}ET_ONE_REG.

Thanks,
Reiji

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