From: Mauro Matteo Cascella <mcascell@redhat.com> To: "Daniel P. Berrangé" <berrange@redhat.com> Cc: "Philippe Mathieu-Daudé" <philmd@linaro.org>, "Bug 1863025" <1863025@bugs.launchpad.net>, qemu-devel@nongnu.org, "Michael Tokarev" <mjt@tls.msk.ru> Subject: Re: [Bug 1863025] Re: Use-after-free after flush in TCG accelerator Date: Thu, 31 Aug 2023 16:10:11 +0200 [thread overview] Message-ID: <CAA8xKjXNFVtzoYaVYz_apW2i3Qbvs0XC4JBS5EG7q_qPTOiAAw@mail.gmail.com> (raw) In-Reply-To: <ZPCcRD/zv/l80WWC@redhat.com> On Thu, Aug 31, 2023 at 3:57 PM Daniel P. Berrangé <berrange@redhat.com> wrote: > > On Thu, Aug 31, 2023 at 03:40:25PM +0200, Philippe Mathieu-Daudé wrote: > > Hi Samuel, > > > > On 31/8/23 14:48, Samuel Henrique wrote: > > > CVE-2020-24165 was assigned to this: > > > https://nvd.nist.gov/vuln/detail/CVE-2020-24165 > > > > > > I had no involvement in the assignment, posting here for reference only. > > > > > > ** CVE added: https://cve.mitre.org/cgi-bin/cvename.cgi?name=2020-24165 > > > > QEMU 4.2.0 was released in 2019. The issue you report > > has been fixed in commit 886cc68943 ("accel/tcg: fix race > > in cpu_exec_step_atomic (bug 1863025)") which is included > > in QEMU v5.0, released in April 2020, more than 3 years ago. > > > > What do you expect us to do here? I'm not sure whether assigning > > CVE for 3 years old code is a good use of engineering time. > > In any case per our stated security policy, we do not consider TCG to > be providing a security boundary between host and guest, and thus bugs > in TCG aren't considered security flaws: > > https://www.qemu.org/docs/master/system/security.html#non-virtualization-use-case Right, and it is clearly indicated in the referenced launchpad bug: 'The security list informed me "This can not be treated as a security issue"'. This adds up to CVE-2022-36648, which is also a mystery to me in terms of CVE assignment and CVSS scoring (rated as Critical). I don't know what's going on with NVD, there must be something wrong on their side. I disputed both CVEs via https://cveform.mitre.org/. > With regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| > -- Mauro Matteo Cascella Red Hat Product Security PGP-Key ID: BB3410B0
WARNING: multiple messages have this Message-ID (diff)
From: Mauro Matteo Cascella <1863025@bugs.launchpad.net> To: qemu-devel@nongnu.org Subject: Re: [Bug 1863025] Re: Use-after-free after flush in TCG accelerator Date: Thu, 31 Aug 2023 14:10:11 -0000 [thread overview] Message-ID: <CAA8xKjXNFVtzoYaVYz_apW2i3Qbvs0XC4JBS5EG7q_qPTOiAAw@mail.gmail.com> (raw) Message-ID: <20230831141011.iy-_t3TC7gI0Ng5AXcjSYFRydkdI6JjJIMtw2GrODDA@z> (raw) In-Reply-To: ZPCcRD/zv/l80WWC@redhat.com On Thu, Aug 31, 2023 at 3:57 PM Daniel P. Berrangé <berrange@redhat.com> wrote: > > On Thu, Aug 31, 2023 at 03:40:25PM +0200, Philippe Mathieu-Daudé wrote: > > Hi Samuel, > > > > On 31/8/23 14:48, Samuel Henrique wrote: > > > CVE-2020-24165 was assigned to this: > > > https://nvd.nist.gov/vuln/detail/CVE-2020-24165 > > > > > > I had no involvement in the assignment, posting here for reference only. > > > > > > ** CVE added: https://cve.mitre.org/cgi-bin/cvename.cgi?name=2020-24165 > > > > QEMU 4.2.0 was released in 2019. The issue you report > > has been fixed in commit 886cc68943 ("accel/tcg: fix race > > in cpu_exec_step_atomic (bug 1863025)") which is included > > in QEMU v5.0, released in April 2020, more than 3 years ago. > > > > What do you expect us to do here? I'm not sure whether assigning > > CVE for 3 years old code is a good use of engineering time. > > In any case per our stated security policy, we do not consider TCG to > be providing a security boundary between host and guest, and thus bugs > in TCG aren't considered security flaws: > > https://www.qemu.org/docs/master/system/security.html#non-virtualization-use-case Right, and it is clearly indicated in the referenced launchpad bug: 'The security list informed me "This can not be treated as a security issue"'. This adds up to CVE-2022-36648, which is also a mystery to me in terms of CVE assignment and CVSS scoring (rated as Critical). I don't know what's going on with NVD, there must be something wrong on their side. I disputed both CVEs via https://cveform.mitre.org/. > With regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| > -- Mauro Matteo Cascella Red Hat Product Security PGP-Key ID: BB3410B0 -- 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: Fix Released 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 => qemu_wait_io_event_common => 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 => cpu_exec => tb_find => 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: B2 => B3 => C3 (same TB as B2) => A3 => A4 (TB freed) => A5 => B2 => B3 (re-allocates TB from B2) => C4 => C5 (freed/reused TB now executing) => 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. To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1863025/+subscriptions
next prev parent reply other threads:[~2023-08-31 14:11 UTC|newest] Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-02-12 22:01 [Bug 1863025] [NEW] Use-after-free after flush in TCG accelerator Yifan 2020-02-14 14:23 ` [Bug 1863025] " Alex Bennée 2020-02-14 14:29 ` Alex Bennée 2020-02-14 14:49 ` [PATCH] accel/tcg: fix race in cpu_exec_step_atomic (bug 1863025) Alex Bennée 2020-02-14 14:49 ` [Bug 1863025] Re: Use-after-free after flush in TCG accelerator Alex Bennée 2020-02-14 15:22 ` [PATCH] accel/tcg: fix race in cpu_exec_step_atomic (bug 1863025) Paolo Bonzini 2020-02-14 23:31 ` Richard Henderson 2020-02-15 0:01 ` Yifan Lu 2020-02-15 0:01 ` [Bug 1863025] " Yifan 2020-02-14 14:51 ` [Bug 1863025] Re: Use-after-free after flush in TCG accelerator Alex Bennée 2020-02-14 18:09 ` Yifan 2020-02-14 18:18 ` Yifan 2020-03-10 9:14 ` Laurent Vivier 2020-04-30 13:43 ` Laurent Vivier 2023-08-31 12:48 ` Samuel Henrique 2023-08-31 13:40 ` Philippe Mathieu-Daudé 2023-08-31 13:57 ` Daniel P. Berrangé 2023-08-31 13:57 ` Daniel Berrange 2023-08-31 14:10 ` Mauro Matteo Cascella [this message] 2023-08-31 14:10 ` Mauro Matteo Cascella 2023-08-31 14:12 ` Mauro Matteo Cascella
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