From nobody Mon Jun 29 17:35:44 2026 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 68E78C433F5 for ; Sat, 5 Feb 2022 08:17:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231174AbiBEIRl (ORCPT ); Sat, 5 Feb 2022 03:17:41 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:47905 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238918AbiBEIRg (ORCPT ); Sat, 5 Feb 2022 03:17:36 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1644049055; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=FpyU/hxiwc/MJkJ493/UIfJTOWcNrra/dJYpmK9ChNI=; b=B9lxYcVnIUVBsu+Nh1PMqiv+SCdA9RtLMaqHVUPF1ZmJS+HK/kEG3sqnyRlA6Odta4JToK qkcqd2GGWRopzn54Pz/SnCQDUys/9nCznlNBrlDQFd9jCLuqPbsYg3M1zX1SlFoPLmITGi 18FITSkDFIMZlBLFC/dgbmRjeN3qFt8= Received: from mail-oo1-f72.google.com (mail-oo1-f72.google.com [209.85.161.72]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-328-JfjTjOKQMO-z3ksIvKw6_w-1; Sat, 05 Feb 2022 03:17:34 -0500 X-MC-Unique: JfjTjOKQMO-z3ksIvKw6_w-1 Received: by mail-oo1-f72.google.com with SMTP id u126-20020a4a5784000000b002e3688852c8so4752448ooa.1 for ; Sat, 05 Feb 2022 00:17:34 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=FpyU/hxiwc/MJkJ493/UIfJTOWcNrra/dJYpmK9ChNI=; b=hpp2xnjceYxx6SXnWV7EIe40ILqn4dylhAlXvE+uh4M14tsqM49FW+QXfoD59ILdBF cr9CvhOi1ErXEYB7fGlavJyPEAL/WHdIdyZiopciIGV/h/R3vPWOGmHPZy3TawLsYAMU 8R9gwbe7+RdibC+00YnJqvODxalepJiZPYNdFFvA5EnHUSlbCS/BXqdrjmpBlKD5qLTM LiZaY0BwL3DIRxMYf/S8izZsB3AQnRLwO5sawD++CUDGZQD5goHT2b2jY2Wvwr1ombUf iYYiOUIe1jcrc/rO3aKgdGtxy9K3jv/gMfbaDUZcLWkDo3GWMBl3QB1CtLkek6d8+ens Wiig== X-Gm-Message-State: AOAM532Pws55xNB80Owyn5r9NYin0DSeKQPzJHft7TZDobV0yi7ozz6e cnTprsE9TGWqfOU2G58YYZXbgfys/n82+WPRVuYWoUb5uTbiMp19FEM6B8Uqu7qNT68uolaZ8UC rxcF7dPttpsHUdlxYusc/3ym1 X-Received: by 2002:a9d:69d1:: with SMTP id v17mr887841oto.220.1644049053552; Sat, 05 Feb 2022 00:17:33 -0800 (PST) X-Google-Smtp-Source: ABdhPJxHZ8nTlggoeWG+zBTopeYthpriHxLKpwl/g0wOeOS85juAdDUPbrfh3I9zvgK9dj3l9+vEyA== X-Received: by 2002:a9d:69d1:: with SMTP id v17mr887837oto.220.1644049053352; Sat, 05 Feb 2022 00:17:33 -0800 (PST) Received: from localhost.localdomain ([2804:431:c7f0:b1af:f10e:1643:81f3:16df]) by smtp.gmail.com with ESMTPSA id bg34sm1708795oob.14.2022.02.05.00.17.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 05 Feb 2022 00:17:32 -0800 (PST) From: Leonardo Bras To: Paolo Bonzini , Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" Cc: Leonardo Bras , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v1 1/2] x86/kvm/fpu: Mask guest fpstate->xfeatures with guest_supported_xcr0 Date: Sat, 5 Feb 2022 05:16:58 -0300 Message-Id: <20220205081658.562208-2-leobras@redhat.com> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20220205081658.562208-1-leobras@redhat.com> References: <20220205081658.562208-1-leobras@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" During host/guest switch (like in kvm_arch_vcpu_ioctl_run()), the kernel swaps the fpu between host/guest contexts, by using fpu_swap_kvm_fpstate(). When xsave feature is available, the fpu swap is done by: - xsave(s) instruction, with guest's fpstate->xfeatures as mask, is used to store the current state of the fpu registers to a buffer. - xrstor(s) instruction, with (fpu_kernel_cfg.max_features & XFEATURE_MASK_FPSTATE) as mask, is used to put the buffer into fpu regs. For xsave(s) the mask is used to limit what parts of the fpu regs will be copied to the buffer. Likewise on xrstor(s), the mask is used to limit what parts of the fpu regs will be changed. The mask for xsave(s), the guest's fpstate->xfeatures, is defined on kvm_arch_vcpu_create(), which (in summary) sets it to all features supported by the cpu which are enabled on kernel config. This means that xsave(s) will save to guest buffer all the fpu regs contents the cpu has enabled when the guest is paused, even if they are not used. This would not be an issue, if xrstor(s) would also do that. xrstor(s)'s mask for host/guest swap is basically every valid feature contained in kernel config, except XFEATURE_MASK_PKRU. Accordingto kernel src, it is instead switched in switch_to() and flush_thread(). Then, the following happens with a host supporting PKRU starts a guest that does not support it: 1 - Host has XFEATURE_MASK_PKRU set. 1st switch to guest, 2 - xsave(s) fpu regs to host fpustate (buffer has XFEATURE_MASK_PKRU) 3 - xrstor(s) guest fpustate to fpu regs (fpu regs have XFEATURE_MASK_PKRU) 4 - guest runs, then switch back to host, 5 - xsave(s) fpu regs to guest fpstate (buffer now have XFEATURE_MASK_PKRU) 6 - xrstor(s) host fpstate to fpu regs. On 5, even though the guest does not support PKRU, it does have the flag set on guest fpstate, which is transferred to userspace via vcpu ioctl KVM_GET_XSAVE. This becomes a problem when the user decides on migrating the above guest to another machine that does not support PKRU: The new host restores guest's fpu regs to as they were before (xrstor(s)), but since the new host don't support PKRU, a general-protection exception ocurs in xrstor(s) and that crashes the guest. This can be solved by making the guest's fpstate->xfeatures only hold values compatible to guest_supported_xcr0. This way, on 5 the only flags saved by xsave(s) will be the ones compatible to guest requirements, and thus there will be no issue during migration. Signed-off-by: Leonardo Bras --- arch/x86/kvm/cpuid.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index 28be02adc669..8ce481cc0f9b 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -296,6 +296,9 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *v= cpu) vcpu->arch.guest_supported_xcr0 =3D cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent= ); =20 + /* Mask out features unsupported by guest */ + vcpu->arch.guest_fpu.fpstate->xfeatures &=3D vcpu->arch.guest_supported_x= cr0; + kvm_update_pv_runtime(vcpu); =20 vcpu->arch.maxphyaddr =3D cpuid_query_maxphyaddr(vcpu); --=20 2.35.1 From nobody Mon Jun 29 17:35:44 2026 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 EF013C433FE for ; Sat, 5 Feb 2022 08:17:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S245506AbiBEIRo (ORCPT ); Sat, 5 Feb 2022 03:17:44 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:23596 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239139AbiBEIRk (ORCPT ); Sat, 5 Feb 2022 03:17:40 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1644049059; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=2+z3ZjKtRxG6T6Z5VKFKBhdA6gfV81DVizdvw4/6PUk=; b=Hbs0BxjmCQN5jvgJQglsslFYmOuJww5tj1apg6xHgwzPo652oc8eGQLz+7Th0Bj0sxGdEs K7gIQvJpMVGo3V7qEhWFZtPGslvZc2nMK81a/vzExYdI8r1aBbMDYdow/oyzjtL7oc3XUw Dc3yhU6rOTYgddr2QgPrjmo47jLLvlY= Received: from mail-oi1-f197.google.com (mail-oi1-f197.google.com [209.85.167.197]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-314-fpmUGEoBOD2xR8RyoxSGQg-1; Sat, 05 Feb 2022 03:17:38 -0500 X-MC-Unique: fpmUGEoBOD2xR8RyoxSGQg-1 Received: by mail-oi1-f197.google.com with SMTP id w8-20020aca6208000000b002c7da950057so5294333oib.5 for ; Sat, 05 Feb 2022 00:17:38 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=2+z3ZjKtRxG6T6Z5VKFKBhdA6gfV81DVizdvw4/6PUk=; b=wa5+jIjrQVcENrvqGXL42Nw9Y0AfaMuLmzWzFiGz3In8R6bMLu1R6/7naB3iz95VQq yAwZgIDjz582spZ1j/+5lvzzFUdkNWuy/U0wvy3HUu2C+crwcan57guxIh6x6e3t47dn 1LwA6UNF/VytVxtn2PIlmuHLR/bODiCtdOOFXKV8jBRlz1yCt3EZURNGCxEKEwzZQdkb tPJ+MNtOOgLmmxUuSmIocxHsRwH3kMQUBsVdWeliwlrho9hyMvfcJIPpFTEvkabGve/W Pnbn5d7GlipGPKdngbRD8BmJ0kdV6rQ1vaie1PKkSlmjF+Pswmc/VgV4qpXN6eYV9D02 ZWgw== X-Gm-Message-State: AOAM533nkGJnrRIbqUmeAxwAIZJD49dQnYrzkkzbY3kNQnbmDoZC7yzN SWTRkN5+7t0znqSETirKPvMbTCNpw6vtRJtInany2oX1iFiE/CHKDz3xoGIxQjBdp/7NC01SzDJ uBsxk/KpqDOtEIMSeWrFjhwRQ X-Received: by 2002:a05:6808:17a9:: with SMTP id bg41mr1270093oib.41.1644049057495; Sat, 05 Feb 2022 00:17:37 -0800 (PST) X-Google-Smtp-Source: ABdhPJz6PMVICTXXaDLc3Yid7XTSlSXktoo6ZqEOP3oQm+lihYzAmAhSDV2aZ6Tt5Yid/SGbl4RWdQ== X-Received: by 2002:a05:6808:17a9:: with SMTP id bg41mr1270088oib.41.1644049057343; Sat, 05 Feb 2022 00:17:37 -0800 (PST) Received: from localhost.localdomain ([2804:431:c7f0:b1af:f10e:1643:81f3:16df]) by smtp.gmail.com with ESMTPSA id bg34sm1708795oob.14.2022.02.05.00.17.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 05 Feb 2022 00:17:36 -0800 (PST) From: Leonardo Bras To: Paolo Bonzini , Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" Cc: Leonardo Bras , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v1 2/2] x86/kvm/fpu: Limit setting guest fpu features based on guest_supported_xcr0 Date: Sat, 5 Feb 2022 05:16:59 -0300 Message-Id: <20220205081658.562208-3-leobras@redhat.com> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20220205081658.562208-1-leobras@redhat.com> References: <20220205081658.562208-1-leobras@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" As of today, if userspace tries to set guest's fpu features to any value (vcpu ioctl: KVM_SET_XSAVE), it is checked against the supported features of the host cpu, and the supported features of KVM. This makes possible to set the guest fpstate with features that were not enabled during guest creation, but are available in the host cpu. This becomes an issue during guest migration, if the target host does not support the given feature: 1 - Create guest vcpu without support to featureA, on a source host that supports it, 2 - Set featureA to guest vcpu, even if it does not support it. It will run just fine, as the current host cpu supports featureA, 3 - Migrate guest to another host, which does not support featureA, 4 - After migration is completed, restoring guest fpustate to fpu regs will cause a general-protection exception, and crash the guest. A way to avoid the issue is by returning error if the user tries to set any feature not enabled during guest creation (guest_supported_xcr0). Signed-off-by: Leonardo Bras --- arch/x86/kvm/x86.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 74b53a16f38a..f4e42de3560a 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -5010,7 +5010,8 @@ static int kvm_vcpu_ioctl_x86_set_xsave(struct kvm_vc= pu *vcpu, =20 return fpu_copy_uabi_to_guest_fpstate(&vcpu->arch.guest_fpu, guest_xsave->region, - supported_xcr0, &vcpu->arch.pkru); + vcpu->arch.guest_supported_xcr0, + &vcpu->arch.pkru); } =20 static void kvm_vcpu_ioctl_x86_get_xcrs(struct kvm_vcpu *vcpu, --=20 2.35.1