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

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Date:	Mon, 16 Jun 2014 16:56:31 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Nadav Amit <nadav.amit@...il.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

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?

Paolo
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