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

[Nadav Amit <nadav.amit@gmail.com>]

On 6/16/14, 2:09 PM, Paolo Bonzini wrote:
> 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.

There are two questions that come in mind:
1. Should we ignore CS.DB? It would make it consistent with 
kvm_hv_hypercall and handle_dr. I think this is the proper behavior.

2. Reading CS.L once and masking all the registers (i.e., changing the 
is_long_mode in kvm_emulate_hypercall to is_64_bit_mode) is likely to be 
more efficient.

Regards,
Nadav