From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1C4B0C433FE for ; Thu, 13 Oct 2022 06:52:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229542AbiJMGwc (ORCPT ); Thu, 13 Oct 2022 02:52:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56858 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229460AbiJMGw3 (ORCPT ); Thu, 13 Oct 2022 02:52:29 -0400 Received: from out0-130.mail.aliyun.com (out0-130.mail.aliyun.com [140.205.0.130]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 51013DF68; Wed, 12 Oct 2022 23:52:26 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R161e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018047192;MF=houwenlong.hwl@antgroup.com;NM=1;PH=DS;RN=10;SR=0;TI=SMTPD_---.Paw6veq_1665643942; Received: from localhost(mailfrom:houwenlong.hwl@antgroup.com fp:SMTPD_---.Paw6veq_1665643942) by smtp.aliyun-inc.com; Thu, 13 Oct 2022 14:52:22 +0800 Date: Thu, 13 Oct 2022 14:52:22 +0800 From: "Hou Wenlong" To: Sean Christopherson Cc: kvm@vger.kernel.org, Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , linux-kernel@vger.kernel.org Subject: Re: [PATCH] KVM: x86: Mark transfer type as X86_TRANSFER_RET when loading CS in iret emulation Message-ID: <20221013065222.GA100679@k08j02272.eu95sqa> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 13, 2022 at 12:34:57AM +0800, Sean Christopherson wrote: > On Wed, Oct 12, 2022, Hou Wenlong wrote: > > When loading code segment descriptor in iret instruction emulation, the > > checks are same as far return instruction emulation, so transfer type > > should be X86_TRANSFER_RET in __load_segment_descriptor(). Although, > > only iret in real mode is implemented now, and no checks are actually > > needed for real mode, it would still be better to mark transfer type as > > X86_TRANSFER_RET. > > It's not strictly a RET though. The RPL vs. DPL checks in __load_segment_descriptor() > might do the right thing, but there's a rather large pile of stuff IRET can do that > RET can't (ignoring the fact that KVM doesn't even emulate FAR RET to outer privilege > levels). > Yes, if EFLAGS.NT is set, IRET performs a task switch instead of far return in Protected Mode. > And __emulate_int_real() also loads CS with X86_TRANSFER_NONE, i.e. KVM still has > a weird path to worry about. > > Rather than make the IRET case slightly less wrong, what about adding a sanity > check in __load_segment_descriptor() that KVM doesn't attempt to load CS in Protected > Mode with X86_TRANSFER_NONE? > OK, sanity check is needed if someone wants to implement IRET in Protected Mode but use load_segment_descriptor(). > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c > index 3b27622d4642..fe735e18c419 100644 > --- a/arch/x86/kvm/emulate.c > +++ b/arch/x86/kvm/emulate.c > @@ -1641,6 +1641,14 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt, > goto exception; > break; > case VCPU_SREG_CS: > + /* > + * KVM uses "none" when loading CS as part of emulating Real > + * Mode exceptions and IRET (handled above). In all other > + * cases, loading CS without a control transfer is a KVM bug. > + */ > + if (WARN_ON_ONCE(transfer == X86_TRANSFER_NONE)) > + goto exception; > + > if (!(seg_desc.type & 8)) > goto exception; > Do I need to prepare this patch or you will add this directly? > > > > No functional change intended. > > > > Signed-off-by: Hou Wenlong > > --- > > arch/x86/kvm/emulate.c | 4 +++- > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c > > index 3b27622d4642..5052eb480068 100644 > > --- a/arch/x86/kvm/emulate.c > > +++ b/arch/x86/kvm/emulate.c > > @@ -2100,6 +2100,7 @@ static int emulate_iret_real(struct x86_emulate_ctxt *ctxt) > > X86_EFLAGS_FIXED; > > unsigned long vm86_mask = X86_EFLAGS_VM | X86_EFLAGS_VIF | > > X86_EFLAGS_VIP; > > + u8 cpl = ctxt->ops->cpl(ctxt); > > > > /* TODO: Add stack limit check */ > > > > @@ -2121,7 +2122,8 @@ static int emulate_iret_real(struct x86_emulate_ctxt *ctxt) > > if (rc != X86EMUL_CONTINUE) > > return rc; > > > > - rc = load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS); > > + rc = __load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS, cpl, > > + X86_TRANSFER_RET, NULL); > > > > if (rc != X86EMUL_CONTINUE) > > return rc; > > -- > > 2.31.1 > >