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charset="utf-8" Content-Transfer-Encoding: quoted-printable Date: Fri, 14 Feb 2020 18:18:43 -0000 From: Yifan 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: Yifan (yifanlu) References: <158154486735.14935.3370403781300872079.malonedeb@soybean.canonical.com> Message-Id: <158170432372.15247.8354921641844688637.malone@soybean.canonical.com> 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: 540c47ae8d92d7d541cacec2c9de5d6bf9de23c1 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" Apologies, the patch got messed up. diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c index c01f59c743..7a9e8c94bd 100644 --- a/accel/tcg/cpu-exec.c +++ b/accel/tcg/cpu-exec.c @@ -238,8 +238,11 @@ void cpu_exec_step_atomic(CPUState *cpu) uint32_t flags; uint32_t cflags =3D 1; uint32_t cf_mask =3D cflags & CF_HASH_MASK; + unsigned flush_count; = if (sigsetjmp(cpu->jmp_env, 0) =3D=3D 0) { +retry: + flush_count =3D tb_flush_count(); tb =3D tb_lookup__cpu_state(cpu, &pc, &cs_base, &flags, cf_mask); if (tb =3D=3D NULL) { mmap_lock(); @@ -248,6 +251,11 @@ void cpu_exec_step_atomic(CPUState *cpu) } = start_exclusive(); + /* do_tb_flush() might run and make tb invalid */ + if (flush_count !=3D tb_flush_count()) { + end_exclusive(); + goto retry; + } = /* Since we got here, we know that parallel_cpus must be true. */ parallel_cpus =3D false; diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c index 9f48da9472..2fb7da9b51 100644 --- a/accel/tcg/translate-all.c +++ b/accel/tcg/translate-all.c @@ -2674,3 +2674,8 @@ void tcg_flush_softmmu_tlb(CPUState *cs) tlb_flush(cs); #endif } + +unsigned tb_flush_count(void) +{ + return atomic_read(&tb_ctx.tb_flush_count); +} diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h index d85e610e85..aa3c2d219a 100644 --- a/include/exec/exec-all.h +++ b/include/exec/exec-all.h @@ -579,6 +579,9 @@ void tlb_set_dirty(CPUState *cpu, target_ulong vaddr); /* exec.c */ void tb_flush_jmp_cache(CPUState *cpu, target_ulong addr); = +/* translate-all.c */ +unsigned tb_flush_count(void); + MemoryRegionSection * address_space_translate_for_iotlb(CPUState *cpu, int asidx, hwaddr addr, hwaddr *xlat, hwaddr *plen, -- = 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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