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 X-Spam-Level: X-Spam-Status: No, score=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 03A44C433DB for ; Wed, 13 Jan 2021 01:03:23 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 1DBC92312F for ; Wed, 13 Jan 2021 01:03:21 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1DBC92312F Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=bugs.launchpad.net Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:53090 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kzUZE-0004Oi-Gr for qemu-devel@archiver.kernel.org; Tue, 12 Jan 2021 20:03:20 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:39308) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kzUX3-000322-Sj for qemu-devel@nongnu.org; Tue, 12 Jan 2021 20:01:07 -0500 Received: from indium.canonical.com ([91.189.90.7]:39622) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1kzUWr-00086c-DH for qemu-devel@nongnu.org; Tue, 12 Jan 2021 20:00:58 -0500 Received: from loganberry.canonical.com ([91.189.90.37]) by indium.canonical.com with esmtp (Exim 4.86_2 #2 (Debian)) id 1kzUWp-0000FO-ES for ; Wed, 13 Jan 2021 01:00:51 +0000 Received: from loganberry.canonical.com (localhost [127.0.0.1]) by loganberry.canonical.com (Postfix) with ESMTP id 6572F2E813C for ; Wed, 13 Jan 2021 01:00:51 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Date: Wed, 13 Jan 2021 00:50:25 -0000 From: Marco <1911351@bugs.launchpad.net> To: qemu-devel@nongnu.org X-Launchpad-Notification-Type: bug X-Launchpad-Bug: product=qemu; status=Confirmed; importance=Undecided; assignee=None; X-Launchpad-Bug-Information-Type: Public X-Launchpad-Bug-Private: no X-Launchpad-Bug-Security-Vulnerability: no X-Launchpad-Bug-Commenters: charco pmaydell X-Launchpad-Bug-Reporter: Marco (charco) X-Launchpad-Bug-Modifier: Marco (charco) References: <161049230444.13717.10732991676985431455.malonedeb@gac.canonical.com> Message-Id: <161049902653.7145.12925166724636821241.launchpad@wampee.canonical.com> Subject: [Bug 1911351] Re: x86-64 MTTCG Does not update page table entries atomically X-Launchpad-Message-Rationale: Subscriber (QEMU) @qemu-devel-ml X-Launchpad-Message-For: qemu-devel-ml Precedence: bulk X-Generated-By: Launchpad (canonical.com); Revision="511b4a3b6512aa3d421c5f7d74f3527e78bff26e"; Instance="production" X-Launchpad-Hash: 096b22e3407f3d1801bbb95c491bd929833e34f3 Received-SPF: none client-ip=91.189.90.7; envelope-from=bounces@canonical.com; helo=indium.canonical.com X-Spam_score_int: -65 X-Spam_score: -6.6 X-Spam_bar: ------ X-Spam_report: (-6.6 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.249, RCVD_IN_DNSWL_HI=-5, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_NONE=0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Bug 1911351 <1911351@bugs.launchpad.net> Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" ** Description changed: It seems like the qemu tcg code for x86-64 doesn't write the access and - dirty flags of the page table entries atomically. Instead, they first + dirty bits of the page table entries atomically. Instead, they first read the entry, see if they need to set the page table entry, and then - overwrite the entry. So if you have two threads running at the same - time, one accessing the virtual address over and over again, and the - other modifying the page table entry, it is possible that after the - second thread modifies the page table entry, qemu overwrites the value - with the old page table entry value, with the access/dirty flags set. + write back the updated page table entry. So if you have two threads + running at the same time, one accessing the virtual address over and + over again, and the other modifying the page table entry, it is possible + that after the second thread modifies the page table entry, qemu + overwrites the value with the old page table entry value, with the + access/dirty flags set. = Here's a unit test that reproduces this behavior: = https://github.com/mvanotti/kvm-unit- tests/commit/09f9722807271226a714b04f25174776454b19cd = You can run it with: = ``` /usr/bin/qemu-system-x86_64 --no-reboot -nodefaults \ -device pc-testdev -device isa-debug-exit,iobase=3D0xf4,iosize=3D0x4 \ -vnc none -serial stdio -device pci-testdev \ -smp 4 -machine q35 --accel tcg,thread=3Dmulti \ -kernel x86/mmu-race.flat # -initrd /tmp/tmp.avvPpezMFf ``` = Expected output (failure): = ``` kvm-unit-tests$ make && /usr/bin/qemu-system-x86_64 --no-reboot -nodefaul= ts -device pc-testdev -device isa-debug-exit,iobase=3D0xf4,iosize=3D0x4 -vn= c none -serial stdio -device pci-testdev -smp 4 -machine q35 --accel tcg,th= read=3Dmulti -kernel x86/mmu-race.flat # -initrd /tmp/tmp.avvPpezMFf enabling apic enabling apic enabling apic enabling apic paging enabled cr0 =3D 80010011 cr3 =3D 627000 cr4 =3D 20 found 4 cpus PASS: Need more than 1 CPU Detected overwritten PTE: - want: 0x000000000062e007 - got: 0x000000000062d027 + =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0want: 0x000000000062e007 + =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0got: 0x000000000062d027 FAIL: PTE not overwritten PASS: All Reads were zero SUMMARY: 3 tests, 1 unexpected failures ``` = - This bug has allows user-to-root privilege escalation inside the guest - VM: if the user is able overwrite an entry that belongs to a second-to- - last level page table, and is able to allocate the referenced page, then - the user would be in control of a last-level page table, being able to - map any memory they want. This is not uncommon in situations where - memory is being decomitted. + This bug allows user-to-root privilege escalation inside the guest VM: + if the user is able overwrite an entry that belongs to a second-to-last + level page table, and is able to allocate the referenced page, then the + user would be in control of a last-level page table, being able to map + any memory they want. This is not uncommon in situations where memory is + being decomitted. -- = You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1911351 Title: x86-64 MTTCG Does not update page table entries atomically Status in QEMU: Confirmed Bug description: It seems like the qemu tcg code for x86-64 doesn't write the access and dirty bits of the page table entries atomically. Instead, they first read the entry, see if they need to set the page table entry, and then write back the updated page table entry. So if you have two threads running at the same time, one accessing the virtual address over and over again, and the other modifying the page table entry, it is possible that after the second thread modifies the page table entry, qemu overwrites the value with the old page table entry value, with the access/dirty flags set. Here's a unit test that reproduces this behavior: https://github.com/mvanotti/kvm-unit- tests/commit/09f9722807271226a714b04f25174776454b19cd You can run it with: ``` /usr/bin/qemu-system-x86_64 --no-reboot -nodefaults \ -device pc-testdev -device isa-debug-exit,iobase=3D0xf4,iosize=3D0x4 \ -vnc none -serial stdio -device pci-testdev \ -smp 4 -machine q35 --accel tcg,thread=3Dmulti \ -kernel x86/mmu-race.flat # -initrd /tmp/tmp.avvPpezMFf ``` Expected output (failure): ``` kvm-unit-tests$ make && /usr/bin/qemu-system-x86_64 --no-reboot -nodefaul= ts -device pc-testdev -device isa-debug-exit,iobase=3D0xf4,iosize=3D0x4 -vn= c none -serial stdio -device pci-testdev -smp 4 -machine q35 --accel tcg,th= read=3Dmulti -kernel x86/mmu-race.flat # -initrd /tmp/tmp.avvPpezMFf enabling apic enabling apic enabling apic enabling apic paging enabled cr0 =3D 80010011 cr3 =3D 627000 cr4 =3D 20 found 4 cpus PASS: Need more than 1 CPU Detected overwritten PTE: =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0want: 0x000000000062e007 =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0got: 0x000000000062d027 FAIL: PTE not overwritten PASS: All Reads were zero SUMMARY: 3 tests, 1 unexpected failures ``` This bug allows user-to-root privilege escalation inside the guest VM: if the user is able overwrite an entry that belongs to a second-to- last level page table, and is able to allocate the referenced page, then the user would be in control of a last-level page table, being able to map any memory they want. This is not uncommon in situations where memory is being decomitted. To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1911351/+subscriptions