From: "Daniel P. Berrangé" <berrange@redhat.com>
To: "Philippe Mathieu-Daudé" <philmd@linaro.org>
Cc: Bug 1863025 <1863025@bugs.launchpad.net>,
qemu-devel@nongnu.org, Michael Tokarev <mjt@tls.msk.ru>,
Mauro Matteo Cascella <mcascell@redhat.com>
Subject: Re: [Bug 1863025] Re: Use-after-free after flush in TCG accelerator
Date: Thu, 31 Aug 2023 14:57:24 +0100 [thread overview]
Message-ID: <ZPCcRD/zv/l80WWC@redhat.com> (raw)
In-Reply-To: <cb2807f0-fc7d-fc03-376e-09c3d5f10a7f@linaro.org>
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
With regards,
Daniel
--
|: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
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WARNING: multiple messages have this Message-ID (diff)
From: Daniel Berrange <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 13:57:24 -0000 [thread overview]
Message-ID: <ZPCcRD/zv/l80WWC@redhat.com> (raw)
Message-ID: <20230831135724.Dwuk5W3VB7NII7qMZi9C4S_BzKDdI4S3CK-sg7B4L4c@z> (raw)
In-Reply-To: 158154486735.14935.3370403781300872079.malonedeb@soybean.canonical.com
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
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 :|
--
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 13:58 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 [this message]
2023-08-31 13:57 ` Daniel Berrange
2023-08-31 14:10 ` Mauro Matteo Cascella
2023-08-31 14:10 ` Mauro Matteo Cascella
2023-08-31 14:12 ` Mauro Matteo Cascella
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