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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS 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 4047BC2BA83 for ; Fri, 14 Feb 2020 15:01:52 +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 1432F24649 for ; Fri, 14 Feb 2020 15:01:51 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1432F24649 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linaro.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:39942 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1j2cTX-0005sq-4v for qemu-devel@archiver.kernel.org; Fri, 14 Feb 2020 10:01:51 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:43205) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1j2cSh-0004YR-KT for qemu-devel@nongnu.org; Fri, 14 Feb 2020 10:01:01 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1j2cSg-0002yf-3e for qemu-devel@nongnu.org; Fri, 14 Feb 2020 10:00:59 -0500 Received: from indium.canonical.com ([91.189.90.7]:53484) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1j2cSf-0002xw-Rw for qemu-devel@nongnu.org; Fri, 14 Feb 2020 10:00:58 -0500 Received: from loganberry.canonical.com ([91.189.90.37]) by indium.canonical.com with esmtp (Exim 4.86_2 #2 (Debian)) id 1j2cSe-00043c-9M for ; Fri, 14 Feb 2020 15:00:56 +0000 Received: from loganberry.canonical.com (localhost [127.0.0.1]) by loganberry.canonical.com (Postfix) with ESMTP id 287652E80CB for ; Fri, 14 Feb 2020 15:00:56 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Date: Fri, 14 Feb 2020 14:49:52 -0000 From: =?utf-8?q?Alex_Benn=C3=A9e?= To: qemu-devel@nongnu.org X-Launchpad-Notification-Type: bug X-Launchpad-Bug: product=qemu; status=Confirmed; importance=Undecided; assignee=alex.bennee@linaro.org; X-Launchpad-Bug-Tags: mttcg tcg X-Launchpad-Bug-Information-Type: Public X-Launchpad-Bug-Private: no X-Launchpad-Bug-Security-Vulnerability: no X-Launchpad-Bug-Commenters: ajbennee yifanlu X-Launchpad-Bug-Reporter: Yifan (yifanlu) X-Launchpad-Bug-Modifier: =?utf-8?q?Alex_Benn=C3=A9e_=28ajbennee=29?= References: <158154486735.14935.3370403781300872079.malonedeb@soybean.canonical.com> Message-Id: <20200214144952.15502-1-alex.bennee@linaro.org> Subject: [Bug 1863025] Re: Use-after-free after flush in TCG accelerator 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="19413b719a8df7423ab1390528edadce9e0e4aca"; Instance="production-secrets-lazr.conf" X-Launchpad-Hash: c4fc7b14c103ddeed418b0021f2943da8288d9a2 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 91.189.90.7 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 1863025 <1863025@bugs.launchpad.net> Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Message-ID: <20200214144952.hG47-ZU9Sr2I7gfLgWBLBe7DhyxZBo2ajDy4vAKW3ec@z> The bug describes a race whereby cpu_exec_step_atomic can acquire a TB which is invalidated by a tb_flush before we execute it. This doesn't affect the other cpu_exec modes as a tb_flush by it's nature can only occur on a quiescent system. The race was described as: B2. tcg_cpu_exec =3D> cpu_exec =3D> tb_find =3D> tb_gen_code B3. tcg_tb_alloc obtains a new TB C3. TB obtained with tb_lookup__cpu_state or tb_gen_code (same TB as B2) A3. start_exclusive critical section entered A4. do_tb_flush is called, TB memory freed/re-allocated A5. end_exclusive exits critical section B2. tcg_cpu_exec =3D> cpu_exec =3D> tb_find =3D> tb_gen_code B3. tcg_tb_alloc reallocates TB from B2 C4. start_exclusive critical section entered C5. cpu_tb_exec executes the TB code that was free in A4 The simplest fix is to widen the exclusive period to include the TB lookup. As a result we can drop the complication of checking we are in the exclusive region before we end it. Signed-off-by: Alex Benn=C3=A9e Cc: Yifan Cc: Bug 1863025 <1863025@bugs.launchpad.net> --- accel/tcg/cpu-exec.c | 21 +++++++++++---------- 1 file changed, 11 insertions(+), 10 deletions(-) diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c index 2560c90eec7..d95c4848a47 100644 --- a/accel/tcg/cpu-exec.c +++ b/accel/tcg/cpu-exec.c @@ -240,6 +240,8 @@ void cpu_exec_step_atomic(CPUState *cpu) uint32_t cf_mask =3D cflags & CF_HASH_MASK; = if (sigsetjmp(cpu->jmp_env, 0) =3D=3D 0) { + start_exclusive(); + tb =3D tb_lookup__cpu_state(cpu, &pc, &cs_base, &flags, cf_mask); if (tb =3D=3D NULL) { mmap_lock(); @@ -247,8 +249,6 @@ void cpu_exec_step_atomic(CPUState *cpu) mmap_unlock(); } = - start_exclusive(); - /* Since we got here, we know that parallel_cpus must be true. */ parallel_cpus =3D false; cc->cpu_exec_enter(cpu); @@ -271,14 +271,15 @@ void cpu_exec_step_atomic(CPUState *cpu) qemu_plugin_disable_mem_helpers(cpu); } = - if (cpu_in_exclusive_context(cpu)) { - /* We might longjump out of either the codegen or the - * execution, so must make sure we only end the exclusive - * region if we started it. - */ - parallel_cpus =3D true; - end_exclusive(); - } + + /* + * As we start the exclusive region before codegen we must still + * be in the region if we longjump out of either the codegen or + * the execution. + */ + g_assert(cpu_in_exclusive_context(cpu)); + parallel_cpus =3D true; + end_exclusive(); } = struct tb_desc { -- = 2.20.1 ** Tags added: mttcg -- = You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1863025 Title: Use-after-free after flush in TCG accelerator Status in QEMU: Confirmed Bug description: I believe I found a UAF in TCG that can lead to a guest VM escape. The security list informed me "This can not be treated as a security issue." and to post it here. I am looking at the 4.2.0 source code. The issue requires a race and I will try to describe it in terms of three concurrent threads. Thread A: A1. qemu_tcg_cpu_thread_fn runs work loop A2. qemu_wait_io_event =3D> qemu_wait_io_event_common =3D> process_queued= _cpu_work A3. start_exclusive critical section entered A4. do_tb_flush is called, TB memory freed/re-allocated A5. end_exclusive exits critical section Thread B: B1. qemu_tcg_cpu_thread_fn runs work loop B2. tcg_cpu_exec =3D> cpu_exec =3D> tb_find =3D> tb_gen_code B3. tcg_tb_alloc obtains a new TB Thread C: C1. qemu_tcg_cpu_thread_fn runs work loop C2. cpu_exec_step_atomic executes C3. TB obtained with tb_lookup__cpu_state or tb_gen_code C4. start_exclusive critical section entered C5. cpu_tb_exec executes the TB code C6. end_exclusive exits critical section Consider the following sequence of events: =C2=A0=C2=A0B2 =3D> B3 =3D> C3 (same TB as B2) =3D> A3 =3D> A4 (TB freed)= =3D> A5 =3D> B2 =3D> =C2=A0=C2=A0B3 (re-allocates TB from B2) =3D> C4 =3D> C5 (freed/reused TB= now executing) =3D> C6 In short, because thread C uses the TB in the critical section, there is no guarantee that the pointer has not been "freed" (rather the memory is marked as re-usable) and therefore a use-after-free occurs. Since the TCG generated code can be in the same memory as the TB data structure, it is possible for an attacker to overwrite the UAF pointer with code generated from TCG. This can overwrite key pointer values and could lead to code execution on the host outside of the TCG sandbox. 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