From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5C78219DF88 for ; Thu, 25 Jul 2024 17:52:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1721929963; cv=none; b=QOkroywNjg5yrVdq+AndE2XfnzFQdkc0uK7Fig7e80PcvRFuu1a7eszcULDpvdyccZ5GG/eJeu5c90+16hdP/hGmUAcVz9LTYxePlgvT6EU6P/1Grwnutjuipp77uDydCUFJJqJzyCu2zKOg6r7pmh5L74yk8uxGWjs7/1dIAFc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1721929963; c=relaxed/simple; bh=Mrq3pVcI6peUqPoMSdpibg7CDGmyuyeGF+0cPsFEeKw=; h=From:To:Cc:Subject:Date:Message-Id:Content-Type:MIME-Version; b=tg7jiXA7zJL1uGypwEYDkl/9VyoSnOuuTAZCkF+eZsSerwj/FPXBOgdda6lb7HNX1FqgpdzJQ4ojL/4ZjK17+YTRYWNbuqMB7rFNH53+UNAGNOaqWxenklM1HijR/ifn85DbrCOQO5rGlvUv+2cY7voEw452Rd6nz7oY725ucuM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=L+/HRP/I; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="L+/HRP/I" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1721929960; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=HhAbBAJ6gL1oMeI4keNegD7Rzp4r1zzadgT6Nf42wKk=; b=L+/HRP/IjuPXW5G1SeQPVUvIi4N9djYVYYdvjlMk1hWWe1FdppzIFpQDY8DwO0TdciFXik JYGRk+BrvizNPldxUewtT1guYDw2+SnnD+Iynnvxrph7SKBKuHVY8SC39T9vsEdU+v1W5V dJ0ptNA0b9MLSYOasK0GeI6L5qt2FVc= Received: from mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-678-A_5hwBBROOCeTCw7g6ibrA-1; Thu, 25 Jul 2024 13:52:37 -0400 X-MC-Unique: A_5hwBBROOCeTCw7g6ibrA-1 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 7A48519560AD; Thu, 25 Jul 2024 17:52:35 +0000 (UTC) Received: from starship.lan (unknown [10.22.8.132]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 0F999300019B; Thu, 25 Jul 2024 17:52:32 +0000 (UTC) From: Maxim Levitsky To: kvm@vger.kernel.org Cc: Sean Christopherson , Dave Hansen , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , linux-kernel@vger.kernel.org, Paolo Bonzini , x86@kernel.org, Maxim Levitsky Subject: [PATCH v3 0/2] Fix for a very old KVM bug in the segment cache Date: Thu, 25 Jul 2024 13:52:30 -0400 Message-Id: <20240725175232.337266-1-mlevitsk@redhat.com> Content-Type: text/plain; charset="utf-8" Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Hi,=0D =0D Recently, while trying to understand why the pmu_counters_test=0D selftest sometimes fails when run nested I stumbled=0D upon a very interesting and old bug:=0D =0D It turns out that KVM caches guest segment state,=0D but this cache doesn't have any protection against concurrent use.=0D =0D This usually works because the cache is per vcpu, and should=0D only be accessed by vCPU thread, however there is an exception:=0D =0D If the full preemption is enabled in the host kernel,=0D it is possible that vCPU thread will be preempted, for=0D example during the vmx_vcpu_reset.=0D =0D vmx_vcpu_reset resets the segment cache bitmask and then initializes=0D the segments in the vmcs, however if the vcpus is preempted in the=0D middle of this code, the kvm_arch_vcpu_put is called which=0D reads SS's AR bytes to determine if the vCPU is in the kernel mode,=0D which caches the old value.=0D =0D Later vmx_vcpu_reset will set the SS's AR field to the correct value=0D in vmcs but the cache still contains an invalid value which=0D can later for example leak via KVM_GET_SREGS and such.=0D =0D In particular, kvm selftests will do KVM_GET_SREGS,=0D and then KVM_SET_SREGS, with a broken SS's AR field passed as is,=0D which will lead to vm entry failure.=0D =0D This issue is not a nested issue, and actually I was able=0D to reproduce it on bare metal, but due to timing it happens=0D much more often nested. The only requirement for this to happen=0D is to have full preemption enabled in the kernel which runs the selftest.=0D =0D pmu_counters_test reproduces this issue well, because it creates=0D lots of short lived VMs, but the issue as was noted=0D about is not related to pmu.=0D =0D To paritally fix this issue, call vmx_segment_cache_clear=0D after we done with segment register setup in vmx_vcpu_reset.=0D =0D V2: incorporated Paolo's suggestion of having=0D vmx_write_segment_cache_start/end functions (thanks!)=0D =0D V3: reverted to a partial fix.=0D =0D Best regards,=0D Maxim Levitsky=0D =0D Maxim Levitsky (2):=0D KVM: nVMX: use vmx_segment_cache_clear=0D VMX: reset the segment cache after segment initialization in=0D vmx_vcpu_reset=0D =0D arch/x86/kvm/vmx/nested.c | 3 ++-=0D arch/x86/kvm/vmx/vmx.c | 10 +++-------=0D arch/x86/kvm/vmx/vmx.h | 5 +++++=0D 3 files changed, 10 insertions(+), 8 deletions(-)=0D =0D -- =0D 2.26.3=0D =0D