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

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Message-ID: <539F243D.6090803@gmail.com>
Date:	Mon, 16 Jun 2014 20:07:09 +0300
From:	Nadav Amit <nadav.amit@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Nadav Amit <namit@...technion.ac.il>
CC:	gleb@...nel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] KVM: x86: check DR6/7 high-bits are clear only on
 long-mode

On 6/16/14, 5:56 PM, Paolo Bonzini wrote:
> Il 16/06/2014 13:53, Nadav Amit ha scritto:
>> 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.
>
> It depends on what you're using it for, but as a start yes.
>
>> 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.
>
> Yes, for the case of kvm_emulate_hypercall.  Then you can build
> kvm_register_readl on top of is_64bit_mode and fix this bug with that
> function.  Did you check that handle_cr is unaffected?

handle_cr is affected, and there are some other instances of affected 
register reads. Actually, most of the vmx instruction exit handling 
routines ignore long-mode and cs.l when considering the register operand 
size.

I'll make the necessary changes and resubmit.

Nadav

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