Skip to content
Fencer Vulns
medium 5.5

CVE-2021-46939 json

tracing: Restructure trace_clock_global() to never block

published 2024-02-27 2 y ago modified 2026-06-17 4 mo ago also EUVD-2021-33587

cvss
5.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 16%
public exploits
0
none known

Summary

In the Linux kernel, the following vulnerability has been resolved:

tracing: Restructure trace_clock_global() to never block

It was reported that a fix to the ring buffer recursion detection would cause a hung machine when performing suspend / resume testing. The following backtrace was extracted from debugging that case:

Call Trace: trace_clock_global+0x91/0xa0 __rb_reserve_next+0x237/0x460 ring_buffer_lock_reserve+0x12a/0x3f0 trace_buffer_lock_reserve+0x10/0x50 __trace_graph_return+0x1f/0x80 trace_graph_return+0xb7/0xf0 ? trace_clock_global+0x91/0xa0 ftrace_return_to_handler+0x8b/0xf0 ? pv_hash+0xa0/0xa0 return_to_handler+0x15/0x30 ? ftrace_graph_caller+0xa0/0xa0 ? trace_clock_global+0x91/0xa0 ? __rb_reserve_next+0x237/0x460 ? ring_buffer_lock_reserve+0x12a/0x3f0 ? trace_event_buffer_lock_reserve+0x3c/0x120 ? trace_event_buffer_reserve+0x6b/0xc0 ? trace_event_raw_event_device_pm_callback_start+0x125/0x2d0 ? dpm_run_callback+0x3b/0xc0 ? pm_ops_is_empty+0x50/0x50 ? platform_get_irq_byname_optional+0x90/0x90 ? trace_device_pm_callback_start+0x82/0xd0 ? dpm_run_callback+0x49/0xc0

With the following RIP:

RIP: 0010:native_queued_spin_lock_slowpath+0x69/0x200

Since the fix to the recursion detection would allow a single recursion to happen while tracing, this lead to the trace_clock_global() taking a spin lock and then trying to take it again:

ring_buffer_lock_reserve() { trace_clock_global() { arch_spin_lock() { queued_spin_lock_slowpath() { /* lock taken */ (something else gets traced by function graph tracer) ring_buffer_lock_reserve() { trace_clock_global() { arch_spin_lock() { queued_spin_lock_slowpath() { /* DEAD LOCK! */

Tracing should *never* block, as it can lead to strange lockups like the above.

Restructure the trace_clock_global() code to instead of simply taking a lock to update the recorded "prev_time" simply use it, as two events happening on two different CPUs that calls this at the same time, really doesn't matter which one goes first. Use a trylock to grab the lock for updating the prev_time, and if it fails, simply try again the next time. If it failed to be taken, that means something else is already updating it.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=212761

Patch / fix

  • Linux Linux 4.4.269, 4.9.269, 4.14.233, 4.19.191, 5.4.118, 5.10.36, 5.11.20, 5.12.3, 5.13
  • linux Debian 14, Debian 13, Debian 12 5.10.38-1

debian advisories DEBIAN-CVE-2021-46939

  • linux Ubuntu 18.04 LTS 4.15.0-151.157
  • linux Ubuntu 20.04 LTS 5.4.0-77.86
  • linux-aws Ubuntu 18.04 LTS 4.15.0-1109.116
  • linux-aws Ubuntu 20.04 LTS 5.4.0-1051.53
  • linux-gcp Ubuntu 20.04 LTS 5.4.0-1046.49
  • linux-kvm Ubuntu 18.04 LTS 4.15.0-1097.99
  • linux-kvm Ubuntu 20.04 LTS 5.4.0-1041.42
  • linux-azure Ubuntu 20.04 LTS 5.4.0-1051.53

ubuntu advisories UBUNTU-CVE-2021-46939

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • kernel-rt RHEL 8 4.18.0-553.16.1.rt7.357.el8_10
  • kernel-rt-kvm RHEL 8 4.18.0-553.16.1.rt7.357.el8_10
  • kernel-rt-core RHEL 8 4.18.0-553.16.1.rt7.357.el8_10
  • kernel-rt-modules RHEL 8 4.18.0-553.16.1.rt7.357.el8_10
  • kernel-rt-debug-kvm RHEL 8 4.18.0-553.16.1.rt7.357.el8_10
  • kernel-rt-debug-core RHEL 8 4.18.0-553.16.1.rt7.357.el8_10
  • kernel-rt-debug-modules RHEL 8 4.18.0-553.16.1.rt7.357.el8_10
  • kernel-rt-modules-extra RHEL 8 4.18.0-553.16.1.rt7.357.el8_10

red hat advisories RHSA-2024:5102RHSA-2024:5101

  • kernel-syms SLES for SAP 12 SP5, SLES 12 SP5 4.12.14-122.216.1
  • kernel-source SLES for SAP 12 SP5, SLES 12 SP5 4.12.14-122.216.1
  • kernel-default SLES for SAP 12 SP5, SLES 12 SP5, SLE Workstation Extension 12 SP5 +2 more 4.12.14-122.216.1
  • kernel-obs-build SLE Software Development Kit 12 SP5 4.12.14-122.216.1
  • kernel-rt SLE Real Time 12 SP5 4.12.14-10.182.1
  • kernel-syms-rt SLE Real Time 12 SP5 4.12.14-10.182.1
  • kernel-rt_debug SLE Real Time 12 SP5 4.12.14-10.182.1
  • kernel-source-rt SLE Real Time 12 SP5 4.12.14-10.182.1

suse advisories SUSE-SU-2024:1870-1SUSE-SU-2024:1646-1SUSE-SU-2024:1643-1SUSE-SU-2024:1489-1SUSE-SU-2024:1465-1SUSE-SU-2024:1454-1

  • kernel AlmaLinux 8 4.18.0-553.16.1.el8_10
  • perf AlmaLinux 8 4.18.0-553.16.1.el8_10
  • bpftool AlmaLinux 8 4.18.0-553.16.1.el8_10
  • kernel-core AlmaLinux 8 4.18.0-553.16.1.el8_10
  • kernel-tools AlmaLinux 8 4.18.0-553.16.1.el8_10
  • python3-perf AlmaLinux 8 4.18.0-553.16.1.el8_10
  • kernel-modules AlmaLinux 8 4.18.0-553.16.1.el8_10
  • kernel-zfcpdump AlmaLinux 8 4.18.0-553.16.1.el8_10

almalinux advisories ALSA-2024:5101ALSA-2024:5102

  • kernel Rocky Linux 8 4.18.0-553.16.1.el8_10.cloud.0.1
  • kernel-rt Rocky Linux 8 4.18.0-553.16.1.rt7.357.el8_10
  • kernel Rocky Linux 8 4.18.0-553.16.1.el8_10

rocky linux advisories RXSA-2024:5101RLSA-2024:5102RLSA-2024:5101

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-46939 updated cvss 5.5

kernel: tracing: Restructure trace_clock_global() to never block

  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux BaseOS (v. 8) perf 4.18.0-553.16.1.el8_10, kernel 4.18.0-553.16.1.el8_10, bpftool 4.18.0-553.16.1.el8_10, kernel-core 4.18.0-553.16.1.el8_10, kernel-tools 4.18.0-553.16.1.el8_10, python3-perf 4.18.0-553.16.1.el8_10, +15 more Red Hat Enterprise Linux CRB (v. 8) kernel-debuginfo-common-x86_64 4.18.0-553.16.1.el8_10, kernel-debuginfo-common-aarch64 4.18.0-553.16.1.el8_10, kernel-debuginfo-common-ppc64le 4.18.0-553.16.1.el8_10
    fixed: vendor fix RHSA-2024:5101
  • Red Hat Enterprise Linux NFV (v. 8) kernel-rt 4.18.0-553.16.1.rt7.357.el8_10, kernel-rt-kvm 4.18.0-553.16.1.rt7.357.el8_10, kernel-rt-core 4.18.0-553.16.1.rt7.357.el8_10, kernel-rt-modules 4.18.0-553.16.1.rt7.357.el8_10, kernel-rt-debug-kvm 4.18.0-553.16.1.rt7.357.el8_10, kernel-rt-debug-core 4.18.0-553.16.1.rt7.357.el8_10, +4 more Red Hat Enterprise Linux RT (v. 8) kernel-rt 4.18.0-553.16.1.rt7.357.el8_10, kernel-rt-core 4.18.0-553.16.1.rt7.357.el8_10, kernel-rt-modules 4.18.0-553.16.1.rt7.357.el8_10, kernel-rt-debug-core 4.18.0-553.16.1.rt7.357.el8_10, kernel-rt-debug-modules 4.18.0-553.16.1.rt7.357.el8_10, kernel-rt-modules-extra 4.18.0-553.16.1.rt7.357.el8_10, +2 more
    fixed: vendor fix RHSA-2024:5102
  • Red Hat Enterprise Linux 9 rv, perf, rtla, kernel, bpftool, libperf, +83 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
18 mo ago

Exploit prediction

0.25% chance of exploitation in the next 30 days, higher than 16% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CISA ADP 3.1 5.5 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
NVD 3.1 5.5 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
CNA 3.1 5.5 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Affected products

  • Linux Linux

Classification

Weakness
  • CWE-400 Uncontrolled Resource Consumption
  • CWE-662 Improper Synchronization
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.