{"id":"CVE-2021-46939","details":"In the Linux kernel, the following vulnerability has been resolved:\n\ntracing: Restructure trace_clock_global() to never block\n\nIt was reported that a fix to the ring buffer recursion detection would\ncause a hung machine when performing suspend / resume testing. The\nfollowing backtrace was extracted from debugging that case:\n\nCall Trace:\n trace_clock_global+0x91/0xa0\n __rb_reserve_next+0x237/0x460\n ring_buffer_lock_reserve+0x12a/0x3f0\n trace_buffer_lock_reserve+0x10/0x50\n __trace_graph_return+0x1f/0x80\n trace_graph_return+0xb7/0xf0\n ? trace_clock_global+0x91/0xa0\n ftrace_return_to_handler+0x8b/0xf0\n ? pv_hash+0xa0/0xa0\n return_to_handler+0x15/0x30\n ? ftrace_graph_caller+0xa0/0xa0\n ? trace_clock_global+0x91/0xa0\n ? __rb_reserve_next+0x237/0x460\n ? ring_buffer_lock_reserve+0x12a/0x3f0\n ? trace_event_buffer_lock_reserve+0x3c/0x120\n ? trace_event_buffer_reserve+0x6b/0xc0\n ? trace_event_raw_event_device_pm_callback_start+0x125/0x2d0\n ? dpm_run_callback+0x3b/0xc0\n ? pm_ops_is_empty+0x50/0x50\n ? platform_get_irq_byname_optional+0x90/0x90\n ? trace_device_pm_callback_start+0x82/0xd0\n ? dpm_run_callback+0x49/0xc0\n\nWith the following RIP:\n\nRIP: 0010:native_queued_spin_lock_slowpath+0x69/0x200\n\nSince the fix to the recursion detection would allow a single recursion to\nhappen while tracing, this lead to the trace_clock_global() taking a spin\nlock and then trying to take it again:\n\nring_buffer_lock_reserve() {\n  trace_clock_global() {\n    arch_spin_lock() {\n      queued_spin_lock_slowpath() {\n        /* lock taken */\n        (something else gets traced by function graph tracer)\n          ring_buffer_lock_reserve() {\n            trace_clock_global() {\n              arch_spin_lock() {\n                queued_spin_lock_slowpath() {\n                /* DEAD LOCK! */\n\nTracing should *never* block, as it can lead to strange lockups like the\nabove.\n\nRestructure the trace_clock_global() code to instead of simply taking a\nlock to update the recorded \"prev_time\" simply use it, as two events\nhappening on two different CPUs that calls this at the same time, really\ndoesn't matter which one goes first. Use a trylock to grab the lock for\nupdating the prev_time, and if it fails, simply try again the next time.\nIf it failed to be taken, that means something else is already updating\nit.\n\n\nBugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=212761","modified":"2026-03-15T22:42:41.050034Z","published":"2024-02-27T19:04:05.873Z","related":["ALSA-2024:5101","ALSA-2024:5102","SUSE-SU-2024:1454-1","SUSE-SU-2024:1465-1","SUSE-SU-2024:1489-1","SUSE-SU-2024:1643-1","SUSE-SU-2024:1646-1","SUSE-SU-2024:1870-1"],"references":[{"type":"FIX","url":"https://git.kernel.org/stable/c/d43d56dbf452ccecc1ec735cd4b6840118005d7c"},{"type":"FIX","url":"https://git.kernel.org/stable/c/6e2418576228eeb12e7ba82edb8f9500623942ff"},{"type":"FIX","url":"https://git.kernel.org/stable/c/859b47a43f5a0e5b9a92b621dc6ceaad39fb5c8b"},{"type":"FIX","url":"https://git.kernel.org/stable/c/91ca6f6a91f679c8645d7f3307e03ce86ad518c4"},{"type":"FIX","url":"https://git.kernel.org/stable/c/a33614d52e97fc8077eb0b292189ca7d964cc534"},{"type":"FIX","url":"https://git.kernel.org/stable/c/aafe104aa9096827a429bc1358f8260ee565b7cc"},{"type":"FIX","url":"https://git.kernel.org/stable/c/1fca00920327be96f3318224f502e4d5460f9545"},{"type":"FIX","url":"https://git.kernel.org/stable/c/2a1bd74b8186d7938bf004f5603f25b84785f63e"},{"type":"FIX","url":"https://git.kernel.org/stable/c/c64da3294a7d59a4bf6874c664c13be892f15f44"}],"affected":[{"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2021-46939.json","unresolved_ranges":[{"events":[{"introduced":"2.6.30"},{"fixed":"4.4.269"}]},{"events":[{"introduced":"4.5.0"},{"fixed":"4.9.269"}]},{"events":[{"introduced":"4.10.0"},{"fixed":"4.14.233"}]},{"events":[{"introduced":"4.15.0"},{"fixed":"4.19.191"}]},{"events":[{"introduced":"4.20.0"},{"fixed":"5.4.118"}]},{"events":[{"introduced":"5.5.0"},{"fixed":"5.10.36"}]},{"events":[{"introduced":"5.11.0"},{"fixed":"5.11.20"}]},{"events":[{"introduced":"5.12.0"},{"fixed":"5.12.3"}]}]}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"}]}