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 13:09:56 +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 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
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