{"id":"DEBIAN-CVE-2021-46939","details":"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","modified":"2026-09-15T09:02:30.438715293Z","published":"2024-02-27T19:04:05.873Z","upstream":["CVE-2021-46939"],"references":[{"type":"ADVISORY","url":"https://security-tracker.debian.org/tracker/CVE-2021-46939"}],"affected":[{"package":{"name":"linux","ecosystem":"Debian:12","purl":"pkg:deb/debian/linux?arch=source&distro=bookworm"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.10.38-1"}]}],"ecosystem_specific":{"urgency":"not yet assigned"},"database_specific":{"source":"https://storage.googleapis.com/osv-test-debian-osv/debian-cve-osv/DEBIAN-CVE-2021-46939.json"}},{"package":{"name":"linux","ecosystem":"Debian:13","purl":"pkg:deb/debian/linux?arch=source&distro=trixie"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.10.38-1"}]}],"ecosystem_specific":{"urgency":"not yet assigned"},"database_specific":{"source":"https://storage.googleapis.com/osv-test-debian-osv/debian-cve-osv/DEBIAN-CVE-2021-46939.json"}},{"package":{"name":"linux","ecosystem":"Debian:14","purl":"pkg:deb/debian/linux?arch=source&distro=forky"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.10.38-1"}]}],"ecosystem_specific":{"urgency":"not yet assigned"},"database_specific":{"source":"https://storage.googleapis.com/osv-test-debian-osv/debian-cve-osv/DEBIAN-CVE-2021-46939.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"}]}