From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH 6/6] KVM: x86: check DR6/7 high-bits are clear only on long-mode Date: Mon, 16 Jun 2014 16:56:31 +0200 Message-ID: <539F059F.8050501@redhat.com> References: <1402837982-24959-1-git-send-email-namit@cs.technion.ac.il> <1402837982-24959-7-git-send-email-namit@cs.technion.ac.il> <539EC43F.607@redhat.com> <539EC7F7.2000208@gmail.com> <539ED084.2000906@redhat.com> <539EDABF.5060701@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Cc: gleb@kernel.org, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: Nadav Amit , Nadav Amit Return-path: In-Reply-To: <539EDABF.5060701@gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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