Re: [PATCH 6/6] KVM: x86: check DR6/7 high-bits are clear only on long-mode

Paolo Bonzini
2014-06-16T11:09:56Z

Il 16/06/2014 12:33, Nadav Amit ha scritto:
>>
>> Do you get this if the input register has bit 31 set?
> No. To be frank, the scenario may be considered a bit synthetic: the
> guest assigns a value to a general-purpose register in 64-bit mode,
> setting the high 32-bits to some non-zero value. Then, later, in 32-bit
> mode, the guest performs MOV DR instruction. In between the two
> assignments, the general purpose register is unmodified, so the high
> 32-bits of the general purpose registers are still set.
>
> Note that this scenario does not occur when MOV DR is emulated, but when
> handle_dr() is called. In this case, the entire 64-bits of the general
> purpose register used for MOV DR are read, regardless to the execution
> mode of the guest.

I wonder if the same bug happens elsewhere. For example,
kvm_emulate_hypercall doesn't look at CS.L/CS.DB, which is really a
corner case but arguably also a bug. kvm_hv_hypercall instead does it
right.

Perhaps we need a variant of kvm_register_read that (on 64-bit hosts)
checks EFER/CS.L/CS.DB and masks the returned value accordingly. You
could call it kvm_register_readl.

Paolo