From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nadav Amit Subject: Re: [PATCH 6/6] KVM: x86: check DR6/7 high-bits are clear only on long-mode Date: Mon, 16 Jun 2014 14:53:35 +0300 Message-ID: <539EDABF.5060701@gmail.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> 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: Paolo Bonzini , Nadav Amit Return-path: In-Reply-To: <539ED084.2000906@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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